Boiler fault code
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Likely Cause

No gas supply, faulty ignition electrode, blocked burner or PCB failure.

What To Do

1. 1

   Check other gas appliances in the property work.

2. 2

   Reset once using the reset button.

3. 3

   If fault returns, do not keep resetting — call a Gas Safe engineer.

Likely Cause

Flame detected then lost mid-operation; draught, dirty ionisation electrode, or gas pressure drop.

What To Do

1. 1

   Do not repeatedly reset.

2. 2

   Check the flue outlet is clear of obstructions.

3. 3

   Call a Gas Safe engineer for ionisation electrode inspection.

Likely Cause

Flue NTC sensor failure or overheating protection triggered.

What To Do

1. 1

   Switch off and allow to cool 30 minutes.

2. 2

   Attempt one reset.

3. 3

   If fault persists, sensor replacement required.

Likely Cause

Primary circuit overheated — pump failure, air lock, or blocked heat exchanger.

What To Do

1. 1

   Allow boiler to cool 30–60 min before resetting.

2. 2

   Ensure all TRVs and zone valves are open.

3. 3

   If reoccurs, book an engineer urgently.

Likely Cause

System pressure below 0.5 bar — leak or failed filling loop.

What To Do

1. 1

   Locate the filling loop under the boiler.

2. 2

   Open valve slowly until gauge reads 1.0–1.5 bar.

3. 3

   Close valve. If pressure drops again within days, a leak is present.

Likely Cause

Circulation pump has failed, seized, or system is air-locked.

What To Do

1. 1

   Do not attempt to open the boiler.

2. 2

   Check all radiator valves are open.

3. 3

   Book an engineer — pump replacement or system flush may be needed.

Likely Cause

Temperature spread between flow and return too high — low flow rate or partial blockage.

What To Do

1. 1

   Check all zone valves and TRVs are open.

2. 2

   Bleed radiators to remove air.

3. 3

   Book an engineer to investigate flow rate.

Likely Cause

Condensate level sensor malfunction or condensate pipe blockage.

What To Do

1. 1

   Check condensate pipe for blockage or freezing (in cold weather).

2. 2

   Clear any blockage and reset.

3. 3

   If sensor is faulty, engineer replacement required.

Likely Cause

Gas valve stuck open or ionisation fault — safety lockout.

What To Do

1. 1

   Do not attempt to reset repeatedly.

2. 2

   Isolate boiler at gas valve immediately.

3. 3

   Call a Gas Safe engineer — gas valve replacement likely needed.

Likely Cause

Flame not established within time window — gas valve, electrode or modulator fault.

What To Do

1. 1

   Check gas supply.

2. 2

   Wait 15 minutes, try one reset.

3. 3

   If recurs, do not keep resetting — engineer required.

Likely Cause

Gas valve fault, low gas pressure, or combustion issue.

What To Do

1. 1

   Do not attempt multiple resets.

2. 2

   Gas Safe engineer required to test gas valve.

3. 3

   Part replacement likely — call for fixed-price quote.

Likely Cause

Fan speed outside expected range — fan motor or hall sensor failure.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Fan inspection or replacement required.

3. 3

   Book a Gas Safe engineer.

Likely Cause

Fan not reaching correct RPM — motor wear, capacitor failure, or debris.

What To Do

1. 1

   One reset attempt only.

2. 2

   Fan motor or capacitor replacement needed.

3. 3

   Call an engineer.

Likely Cause

Loss of communication between boiler and external controls (programmer, room stat).

What To Do

1. 1

   Check external controls are powered and connected.

2. 2

   Inspect eBUS wiring connections at boiler and controller.

3. 3

   Try one reset. If fault persists, engineer required.

Likely Cause

Return NTC thermistor failure or wiring fault.

What To Do

1. 1

   One reset.

2. 2

   If fault recurs, sensor or wiring inspection needed.

3. 3

   Book an engineer.

Likely Cause

Short circuit or open circuit on gas valve driver stage of PCB.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Engineer required — PCB or gas valve diagnosis.

3. 3

   Isolate boiler and call immediately.

Likely Cause

Gas valve taking too long to close after shutdown — valve or safety circuit issue.

What To Do

1. 1

   Isolate boiler at gas cock.

2. 2

   Do not attempt to restart.

3. 3

   Gas Safe engineer required immediately.

Likely Cause

Internal PCB memory error.

What To Do

1. 1

   Try cycling power at mains (off 5 min, on).

2. 2

   If fault persists, PCB replacement required.

3. 3

   Call an engineer.

Likely Cause

NTC sensor(s) shorted — simultaneous flow and return sensor fault.

What To Do

1. 1

   Engineer required — do not attempt to reset.

2. 2

   Both sensors or PCB connections need inspection.

3. 3

   Call us for a diagnosis.

Likely Cause

PCB or component overheating — ventilation issue or PCB fault.

What To Do

1. 1

   Ensure boiler casing is properly fitted and ventilation slots are clear.

2. 2

   Switch off and call an engineer.

3. 3

   PCB may need replacement.

Likely Cause

Pressure sensor giving incorrect reading or has failed.

What To Do

1. 1

   Check system pressure on the gauge — if correct, sensor is likely faulty.

2. 2

   One reset attempt.

3. 3

   Sensor replacement if fault persists.

Likely Cause

Failed to establish flame — gas supply, electrode, or PCB fault.

What To Do

1. 1

   Check other gas appliances work.

2. 2

   Hold reset button 3 seconds.

3. 3

   If EA returns after 2 attempts, call an engineer immediately.

Likely Cause

System pressure below 0.8 bar.

What To Do

1. 1

   Re-pressurise via filling loop to 1.0–1.5 bar.

2. 2

   Reset.

3. 3

   If pressure drops again quickly, investigate for a leak.

Likely Cause

Flow temperature NTC sensor failure.

What To Do

1. 1

   One reset.

2. 2

   If fault returns, NTC sensor replacement required.

3. 3

   Book an engineer.

Likely Cause

Boiler overheated; low flow, pump fault, or blocked heat exchanger.

What To Do

1. 1

   Allow to cool fully before resetting.

2. 2

   Open all TRVs and zone valves.

3. 3

   Book engineer if reoccurs.

Likely Cause

Boiler has exceeded safe temperature — pump failure, low flow, or blocked HX.

What To Do

1. 1

   Allow to cool 30–60 min.

2. 2

   Check all valves open.

3. 3

   Book engineer urgently if reoccurs.

Likely Cause

Fan not reaching or maintaining correct RPM.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Fan or PCB inspection required.

3. 3

   Call a Gas Safe engineer.

Likely Cause

Fan driver circuit on PCB has failed.

What To Do

1. 1

   Engineer required — do not keep resetting.

2. 2

   PCB or fan replacement needed.

3. 3

   Call us.

Likely Cause

Return NTC thermistor failure.

What To Do

1. 1

   One reset.

2. 2

   If fault returns, sensor replacement required.

3. 3

   Book an engineer.

Likely Cause

Gas valve not opening correctly.

What To Do

1. 1

   Do not attempt multiple resets.

2. 2

   Gas Safe engineer to test gas valve.

3. 3

   Part replacement likely.

Likely Cause

Open circuit on gas valve wiring loom.

What To Do

1. 1

   Do not reset.

2. 2

   Engineer required to inspect valve wiring.

3. 3

   Call us.

Likely Cause

Domestic hot water NTC sensor failure.

What To Do

1. 1

   One reset.

2. 2

   DHW NTC sensor replacement needed.

3. 3

   Book an engineer.

Likely Cause

Flue gas temperature sensor has failed.

What To Do

1. 1

   Allow to cool. Try one reset.

2. 2

   Sensor replacement required if fault persists.

3. 3

   Book an engineer.

Likely Cause

Insufficient gas pressure at inlet — supply issue or gas valve fault.

What To Do

1. 1

   Check other gas appliances work.

2. 2

   If supply is OK, gas valve inspection needed.

3. 3

   Call an engineer.

Likely Cause

Failed to detect flame — gas supply, electrode or PCB issue.

What To Do

1. 1

   Check gas supply.

2. 2

   Reset once.

3. 3

   If E1 returns, call an engineer.

Likely Cause

Flame detected then lost — gas pressure or dirty ionisation electrode.

What To Do

1. 1

   Check other gas appliances.

2. 2

   Try one reset.

3. 3

   Persistent E2 needs engineer — flame sensor or gas valve.

Likely Cause

Primary circuit overheated — pump failure, air lock, or blocked HX.

What To Do

1. 1

   Cool fully before resetting.

2. 2

   Open all valves.

3. 3

   Recurrence needs engineer urgently.

Likely Cause

Flue temperature sensor indicating excessive temperature.

What To Do

1. 1

   Cool boiler. Try one reset.

2. 2

   If reoccurs, flue sensor or combustion issue needs engineer.

3. 3

   Do not delay.

Likely Cause

Flow temperature thermistor failure.

What To Do

1. 1

   One reset.

2. 2

   Sensor replacement required if fault persists.

3. 3

   Book an engineer.

Likely Cause

Return temperature thermistor failure.

What To Do

1. 1

   One reset.

2. 2

   Sensor replacement required.

3. 3

   Call an engineer.

Likely Cause

Domestic hot water temperature sensor failure.

What To Do

1. 1

   One reset.

2. 2

   Engineer required for sensor replacement.

3. 3

   Book a visit.

Likely Cause

System pressure below safe operating threshold.

What To Do

1. 1

   Re-pressurise via filling loop to 1.0–1.5 bar.

2. 2

   Reset boiler.

3. 3

   Persistent pressure loss = leak — call us.

Likely Cause

Fan RPM outside acceptable range — motor or PCB fault.

What To Do

1. 1

   Do not keep resetting.

2. 2

   Fan or PCB inspection required.

3. 3

   Call a Gas Safe engineer.

Likely Cause

Open or short circuit in gas valve wiring.

What To Do

1. 1

   Do not reset.

2. 2

   Engineer needed to inspect gas valve and wiring.

3. 3

   Call us.

Likely Cause

Electronic control board fault or internal sensor failure.

What To Do

1. 1

   Power off at mains 10 min, restart.

2. 2

   If fault persists, PCB may need replacing.

3. 3

   Call us.

Likely Cause

Gas valve relay on the PCB has failed, or the gas valve itself is not responding correctly. Also seen on Potterton and Main Heating models sharing the same PCB platform.

What To Do

1. 1

   Do not attempt to reset repeatedly.

2. 2

   Isolate the boiler at the gas cock.

3. 3

   A Gas Safe engineer must inspect the gas valve and PCB relay — do not restart until assessed.

Likely Cause

Gas valve not operating within parameters.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Engineer required for gas valve testing.

3. 3

   Call us.

Likely Cause

Gas supply interrupted or ignition failure.

What To Do

1. 1

   Check other gas appliances.

2. 2

   Reset once.

3. 3

   If F1 returns, call an engineer — do not keep resetting.

Likely Cause

Flame established then extinguished — gas valve or ionisation fault.

What To Do

1. 1

   Try one reset.

2. 2

   Recurrence needs professional inspection of valve and sensor.

3. 3

   Do not reset more than twice.

Likely Cause

Flue gas thermostat has tripped or failed.

What To Do

1. 1

   Cool boiler. Try one reset.

2. 2

   Check flue for obstruction.

3. 3

   Engineer required if reoccurs.

Likely Cause

Flow temperature thermistor failure.

What To Do

1. 1

   One reset.

2. 2

   Sensor replacement required.

3. 3

   Book an engineer.

Likely Cause

Return temperature thermistor failure.

What To Do

1. 1

   One reset.

2. 2

   Sensor inspection and replacement needed.

3. 3

   Book us.

Likely Cause

Hot water NTC sensor failure.

What To Do

1. 1

   One reset.

2. 2

   DHW sensor replacement needed.

3. 3

   Book an engineer.

Likely Cause

Gas valve not opening or closing correctly.

What To Do

1. 1

   Do not reset.

2. 2

   Gas Safe engineer required.

3. 3

   Call us immediately.

Likely Cause

Insufficient gas pressure reaching boiler.

What To Do

1. 1

   Check other gas appliances.

2. 2

   If supply OK, gas valve or governor inspection needed.

3. 3

   Call us.

Likely Cause

Boiler exceeded safe temperature — pump, flow, or HX issue.

What To Do

1. 1

   Cool 30 min before resetting.

2. 2

   Open all radiator valves.

3. 3

   Reoccurrence requires urgent engineer visit.

Likely Cause

Hot water temperature exceeded safe limit — DHW NTC fault or thermistor issue.

What To Do

1. 1

   Cool boiler.

2. 2

   One reset.

3. 3

   Engineer required if fault reoccurs.

Likely Cause

Air pressure switch not making — fan issue or blocked flue.

What To Do

1. 1

   Check flue for obstruction.

2. 2

   Do not reset repeatedly.

3. 3

   Engineer required.

Likely Cause

Ignition failure — gas supply, electrode, or PCB fault.

What To Do

1. 1

   Check gas supply.

2. 2

   One reset attempt.

3. 3

   If recurs, call a specialist engineer.

Likely Cause

Flow NTC thermistor failure.

What To Do

1. 1

   One reset.

2. 2

   Sensor replacement required.

3. 3

   Call an engineer.

Likely Cause

Return NTC thermistor failure.

What To Do

1. 1

   One reset.

2. 2

   Sensor replacement required.

3. 3

   Book an engineer.

Likely Cause

Fan not reaching correct RPM.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Fan inspection or replacement needed.

3. 3

   Call a Viessmann specialist.

Likely Cause

Temperature exceeded safety limit — pump, flow or HX fault.

What To Do

1. 1

   Cool fully before resetting.

2. 2

   Open all valves.

3. 3

   Reoccurrence needs urgent engineer visit.

Likely Cause

Annual service interval reached.

What To Do

1. 1

   Book an annual service with a qualified engineer.

2. 2

   Reset service indicator after service is complete.

3. 3

   Do not ignore — maintains warranty and efficiency.

Likely Cause

Communication error between boiler and external controls/cascade.

What To Do

1. 1

   Check wiring to external controls.

2. 2

   Inspect Vitotronic connections.

3. 3

   Engineer required if wiring is intact.

Likely Cause

External temperature sensor not correctly fitted or wiring fault.

What To Do

1. 1

   Check the sensor is correctly mounted outside.

2. 2

   Check wiring connections at the sensor and boiler.

3. 3

   Call an engineer if wiring is intact but fault persists.

Likely Cause

Primary boiler temperature sensor not correctly fitted or connection fault.

What To Do

1. 1

   Check the sensor is correctly fitted and connected.

2. 2

   One reset.

3. 3

   If fault persists, sensor or wiring replacement required — call an engineer.

Likely Cause

Flue gas temperature sensor not correctly fitted or connected.

What To Do

1. 1

   Check sensor is correctly fitted in the flue.

2. 2

   Check wiring connections.

3. 3

   Call an engineer if fault persists.

Likely Cause

Secondary flow temperature sensor not correctly fitted or wiring fault.

What To Do

1. 1

   Check sensor is correctly fitted and connected.

2. 2

   One reset.

3. 3

   Sensor or wiring replacement if fault persists — call an engineer.

Likely Cause

Return temperature sensor not correctly fitted or connection fault.

What To Do

1. 1

   Check sensor is correctly fitted and connected.

2. 2

   One reset.

3. 3

   Call an engineer if fault persists.

Likely Cause

DHW temperature sensor not correctly fitted or wiring fault.

What To Do

1. 1

   Check sensor is correctly fitted on DHW outlet or cylinder.

2. 2

   Check wiring connections.

3. 3

   Call an engineer if fault persists.

Likely Cause

Room thermostat or controller not communicating correctly with the boiler.

What To Do

1. 1

   Check the boiler wiring connections to the room device.

2. 2

   Check the room device is powered.

3. 3

   Call an engineer if boiler connections are intact.

Likely Cause

Communication error between room device and boiler, or radio clock signal fault.

What To Do

1. 1

   Check compatibility of the room device or clock with the boiler.

2. 2

   Check wiring connections.

3. 3

   Call an engineer or replace incompatible device.

Likely Cause

Pressure sensor not connected correctly or sensor failure.

What To Do

1. 1

   Check the wiring connections to the pressure sensor.

2. 2

   One reset.

3. 3

   If fault persists, sensor replacement needed — call an engineer.

Likely Cause

LPB bus wiring short circuit or bus supply voltage missing.

What To Do

1. 1

   Check the LPB bus wiring for damage or short circuits.

2. 2

   Check wiring connections at boiler and all connected devices.

3. 3

   Call an engineer.

Likely Cause

Two devices on the LPB bus have the same address — configuration error.

What To Do

1. 1

   Check the addressing of all LPB bus devices.

2. 2

   Each device must have a unique address.

3. 3

   Call an engineer to correct the addressing.

Likely Cause

LMU controller has lost stored configuration data — LMU replacement required.

What To Do

1. 1

   Do not attempt to reset repeatedly.

2. 2

   An engineer is required to replace the LMU controller.

3. 3

   Call us immediately.

Likely Cause

Internal fault detected in the LMU (burner management unit) controller.

What To Do

1. 1

   Do not attempt to reset repeatedly.

2. 2

   LMU replacement required — call an engineer.

3. 3

   Call us.

Likely Cause

More than one device on the LPB bus is set as master clock — configuration error.

What To Do

1. 1

   Check parameter 96 of the QAA73 — only one device should have the QAA73 master clock setting.

2. 2

   Correct the configuration.

3. 3

   Call an engineer if unsure.

Likely Cause

Service interval has been reached — maintenance required.

What To Do

1. 1

   Book an annual service with a qualified engineer.

2. 2

   Check the maintenance code value in QAA73 setting 726.

3. 3

   Reset the service indicator after the service is complete.

Likely Cause

Safety temperature limiter (STB) has tripped — water flow insufficient or pump fault.

What To Do

1. 1

   Allow boiler to cool fully.

2. 2

   Check all zone valves and pump are working.

3. 3

   Call a Gas Safe engineer — do not reset without investigating cause.

Likely Cause

Safety thermostat has activated — water flow insufficient, pump fault, or thermostat failure.

What To Do

1. 1

   Allow boiler to cool.

2. 2

   Check all valves are open and pump is running.

3. 3

   Call a Gas Safe engineer — do not reset without inspecting the system.

Likely Cause

Flue gas temperature has exceeded maximum — boiler thermal overload or blocked heat exchanger.

What To Do

1. 1

   Switch off boiler and allow to cool.

2. 2

   Check flue for obstruction.

3. 3

   Call an engineer — heat exchanger may be blocked or boiler is overloading.

Likely Cause

System pressure exceeds 4 bar — expansion vessel failure or filling loop stuck open.

What To Do

1. 1

   Do not use the boiler until pressure is reduced below 3 bar.

2. 2

   Check the filling loop is closed.

3. 3

   Call an engineer — the expansion vessel or PRV may need replacing.

Likely Cause

System pressure below 0.4 bar — low water, leak, or pressure sensor fault.

What To Do

1. 1

   Re-pressurise system to 1.0–1.5 bar via filling loop.

2. 2

   Reset.

3. 3

   If pressure drops repeatedly, a leak is present — call an engineer.

Likely Cause

Flame lost while boiler was running — gas valve fault, ionisation electrode issue, or possible live-neutral inversion on transformer supply.

What To Do

1. 1

   Check gas supply.

2. 2

   One reset attempt.

3. 3

   If fault recurs, call a Gas Safe engineer — do not keep resetting.

Likely Cause

Insufficient combustion air — fan fault, blocked air intake, or flue restriction.

What To Do

1. 1

   Check air inlet and flue for obstruction.

2. 2

   Do not reset repeatedly.

3. 3

   Call a Gas Safe engineer — fan or flue inspection required.

Likely Cause

Flue gas temperature too high — boiler thermal overload or clogged heat exchanger.

What To Do

1. 1

   Switch off and allow to cool.

2. 2

   Check flue is clear.

3. 3

   Call an engineer — heat exchanger inspection needed.

Likely Cause

External safety device (shunt X10-03) has activated — wiring or safety circuit fault.

What To Do

1. 1

   Check that shunt X10-03 is present and wires are properly connected.

2. 2

   Do not reset until cause is investigated.

3. 3

   Call an engineer.

Likely Cause

Ignition attempted but flame not established — gas supply issue, gas valve fault, electrode or ionisation problem.

What To Do

1. 1

   Check gas reaches the boiler — test other gas appliances.

2. 2

   One reset attempt only.

3. 3

   If fault recurs, call a Gas Safe engineer immediately — do not keep resetting.

Likely Cause

Fan speed is below minimum threshold — fan motor wear, wiring, or LMU fault.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Fan inspection required.

3. 3

   Call a Geminox engineer.

Likely Cause

Fan speed is above maximum threshold — fan motor fault or mains supply issue.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Check mains supply voltage.

3. 3

   Call a Geminox engineer — fan or LMU inspection needed.

Likely Cause

New settings have been loaded via QAA73 or PC Tool — a reset is required to validate and return to normal operation.

What To Do

1. 1

   Press reset to validate the new settings.

2. 2

   Boiler should return to normal operation after reset.

3. 3

   Call an engineer if boiler does not resume normal operation.

Likely Cause

Ignition failure — Keston units require specialist knowledge.

What To Do

1. 1

   Try one reset.

2. 2

   If recurs, call a Keston specialist — do not repeatedly reset.

3. 3

   Contact us.

Likely Cause

Flame established then lost — ionisation or gas issue.

What To Do

1. 1

   One reset.

2. 2

   If recurs, engineer required for ionisation and gas valve check.

3. 3

   Call us.

Likely Cause

Overtemperature condition — low flow or HX fault.

What To Do

1. 1

   Cool 30 min before resetting.

2. 2

   Open all valves.

3. 3

   Reoccurrence needs specialist engineer.

Likely Cause

Fan speed fault.

What To Do

1. 1

   One reset.

2. 2

   Fan inspection required if fault persists.

3. 3

   Call a Keston engineer.

Likely Cause

System pressure too low.

What To Do

1. 1

   Re-pressurise to 1.0–1.5 bar.

2. 2

   Reset.

3. 3

   Repeated loss indicates a leak.

Likely Cause

Flow NTC thermistor failure.

What To Do

1. 1

   One reset.

2. 2

   Sensor replacement required.

3. 3

   Book an engineer.

Likely Cause

Failed ignition — gas supply or electrode fault.

What To Do

1. 1

   Check gas supply.

2. 2

   Try one reset.

3. 3

   If F01 returns, call a Gas Safe engineer.

Likely Cause

Flame lost during operation.

What To Do

1. 1

   One reset.

2. 2

   Engineer required for persistent fault.

3. 3

   Call us.

Likely Cause

Boiler overheated — pump or flow issue.

What To Do

1. 1

   Cool fully.

2. 2

   Open all valves.

3. 3

   Reoccurrence needs engineer urgently.

Likely Cause

Flow NTC thermistor failure.

What To Do

1. 1

   One reset.

2. 2

   Sensor replacement required.

3. 3

   Book an engineer.

Likely Cause

Return NTC thermistor failure.

What To Do

1. 1

   One reset.

2. 2

   Sensor replacement required.

3. 3

   Call us.

Likely Cause

System pressure has dropped.

What To Do

1. 1

   Re-pressurise via filling loop to 1.0–1.5 bar.

2. 2

   Reset.

3. 3

   Repeated pressure loss = leak — call us.

Likely Cause

Fan speed fault detected.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Fan inspection required.

3. 3

   Call an engineer.

Likely Cause

Outside temperature sensor open circuit, short circuit, or reading outside of range.

What To Do

1. 1

   Check the outside sensor is correctly fitted and connected.

2. 2

   Inspect wiring for damage.

3. 3

   Call an ATAG engineer if fault persists.

Likely Cause

Flow temperature sensor open circuit, short circuit, or reading outside of range.

What To Do

1. 1

   One reset.

2. 2

   Check sensor wiring connections.

3. 3

   Call an ATAG engineer if fault persists — sensor replacement likely.

Likely Cause

Return temperature sensor open circuit, short circuit, or reading outside of range.

What To Do

1. 1

   One reset.

2. 2

   Check sensor wiring connections.

3. 3

   Call an ATAG engineer if fault persists — sensor replacement likely.

Likely Cause

Connector X7 not connected correctly or low voltage wiring harness defect.

What To Do

1. 1

   Check connector X7 is fully seated and undamaged.

2. 2

   Inspect the low voltage wiring harness for damage.

3. 3

   Call an ATAG engineer.

Likely Cause

Water pressure sensor reading outside range, sensor not connected, or pump has failed.

What To Do

1. 1

   Check system pressure on the gauge.

2. 2

   If pressure is correct, the sensor or pump needs inspection.

3. 3

   Call an ATAG engineer.

Likely Cause

Automatic air venting cycle is running — normal after filling or recommissioning.

What To Do

1. 1

   Allow the venting programme to complete — this is normal.

2. 2

   If it persists beyond 15 minutes, call an engineer.

3. 3

   Do not interrupt the cycle.

Likely Cause

Boiler has reached safety temperature limit — insufficient water flow, pump fault, or blocked heat exchanger.

What To Do

1. 1

   Allow boiler to cool fully before resetting.

2. 2

   Ensure all zone valves and TRVs are open.

3. 3

   Call an ATAG engineer if fault reoccurs — do not keep resetting.

Likely Cause

Flow temperature has exceeded maximum set point — pump, flow, or control fault.

What To Do

1. 1

   Allow to cool.

2. 2

   One reset.

3. 3

   If fault reoccurs, call an ATAG engineer urgently.

Likely Cause

System pressure exceeds 3 bar, or pump is generating excessive pressure — expansion vessel or PRV fault.

What To Do

1. 1

   Do not use boiler until pressure is reduced below 3 bar.

2. 2

   Check the filling loop is fully closed.

3. 3

   Call an engineer — expansion vessel or PRV inspection needed.

Likely Cause

System pressure below 0.7 bar, pump pressure increase too low, or no pump detected.

What To Do

1. 1

   Re-pressurise via filling loop to 1.0–1.5 bar.

2. 2

   Reset.

3. 3

   If pressure drops again or no pump is detected, call an ATAG engineer.

Likely Cause

Required link/jumper between terminals 4 and 5 on connector X2 is absent — wiring configuration error.

What To Do

1. 1

   An engineer must check connector X2 wiring configuration.

2. 2

   Do not attempt to add links without knowing the correct wiring.

3. 3

   Call an ATAG specialist.

Likely Cause

Fan failed to start or reach minimum speed on startup.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Fan inspection required.

3. 3

   Call an ATAG engineer.

Likely Cause

Boiler attempted ignition 5 times without establishing a flame — gas supply, electrode, or gas valve fault.

What To Do

1. 1

   Check other gas appliances to confirm gas supply.

2. 2

   One reset attempt only.

3. 3

   Call a Gas Safe engineer immediately — do not keep resetting.

Likely Cause

Fan speed control not achieved or outside of range, or error in the control unit.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Fan and control unit inspection required.

3. 3

   Call an ATAG engineer.

Likely Cause

Parameter 555 b0 has been changed from 0 to 1 instead of b2 — installer configuration error.

What To Do

1. 1

   This is an installer-level parameter fault.

2. 2

   An ATAG engineer must correct parameter settings.

3. 3

   Change back via green button function (installer level Para 681 b7).

Likely Cause

Flow temperature rising too quickly with a large difference between flow and return — low water flow rate or circulation issue.

What To Do

1. 1

   Check all zone valves and TRVs are open.

2. 2

   Bleed radiators to remove air.

3. 3

   Call an engineer if fault persists — pump or system balance issue.

Likely Cause

Circulation pump has failed or is not running.

What To Do

1. 1

   Do not attempt to open the boiler.

2. 2

   Check all radiator valves are open.

3. 3

   Call an ATAG engineer — pump inspection or replacement required.

Likely Cause

The boiler detected a flame during operation, but it has gone out — ionisation probe, gas valve, or draught issue.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Check gas supply to the boiler.

3. 3

   Call a Gas Safe commercial engineer — ionisation probe and gas valve inspection required.

Likely Cause

The safety temperature limiter (STB) has tripped due to excessive heat — low water flow, pump failure, or blocked HX.

What To Do

1. 1

   Allow boiler to cool fully before resetting.

2. 2

   Ensure all secondary circuit valves are open.

3. 3

   Call a Gas Safe engineer — do not restart without investigating cause.

Likely Cause

System water pressure has dropped below the safe operating level — leak, failed expansion vessel, or PRV weeping.

What To Do

1. 1

   Check system pressure gauge.

2. 2

   Re-pressurise via filling loop to 1.0–1.5 bar if appropriate.

3. 3

   Call a Gas Safe engineer — commercial systems require qualified inspection.

Likely Cause

The combustion fan is not operating within expected parameters — fan motor failure, wiring fault, or PCB issue.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Fan inspection required.

3. 3

   Call a Gas Safe commercial engineer.

Likely Cause

Safety shutdown has occurred due to high temperatures — pump failure, low flow, or blocked heat exchanger.

What To Do

1. 1

   Allow boiler to cool fully.

2. 2

   Open all secondary valves and check pump.

3. 3

   Call a Gas Safe engineer — manual lockout reset required after fault investigation.

Likely Cause

Water pressure switch has cut out — system pressure too low or pressure switch failure.

What To Do

1. 1

   Check system pressure gauge.

2. 2

   Re-pressurise if below 1.0 bar.

3. 3

   Call an engineer if pressure is correct but fault persists — pressure switch may have failed.

Likely Cause

Safety shutdown triggered by gas pressure switch — insufficient gas supply pressure or gas pressure switch fault.

What To Do

1. 1

   Check other gas appliances in the building.

2. 2

   Do not reset repeatedly.

3. 3

   Call a Gas Safe commercial engineer — gas supply pressure must be verified under load.

Likely Cause

Safety time for establishing a flame was exceeded or ignition failed — gas supply, electrode, or gas valve fault.

What To Do

1. 1

   Check gas supply to the boiler.

2. 2

   One reset attempt only.

3. 3

   If fault recurs, call a Gas Safe engineer immediately — do not keep resetting.

Likely Cause

The boiler has been manually locked out — requires physical reset by a qualified engineer after fault investigation.

What To Do

1. 1

   Do not attempt to reset without investigating the underlying cause.

2. 2

   Call a Gas Safe commercial engineer.

3. 3

   Identify the root cause before resetting the lockout.

Likely Cause

Gas supply, ignition electrode, or PCB fault.

What To Do

1. 1

   Check gas supply.

2. 2

   Reset once.

3. 3

   If E1 returns, call an engineer.

Likely Cause

Flame established then lost — gas pressure or ionisation fault.

What To Do

1. 1

   One reset.

2. 2

   Persistent fault needs professional inspection.

3. 3

   Call us.

Likely Cause

System pressure below safe operating level.

What To Do

1. 1

   Re-pressurise via filling loop to 1.0–1.5 bar.

2. 2

   Reset.

3. 3

   Repeat loss = leak — call us.

Likely Cause

Fan speed outside acceptable range.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Fan inspection required.

3. 3

   Call an engineer.

Likely Cause

Boiler overheated — pump, flow, or HX issue.

What To Do

1. 1

   Cool 30 min before resetting.

2. 2

   Open all valves.

3. 3

   Reoccurrence = urgent engineer visit.

Likely Cause

Failed ignition — gas supply, electrode, or PCB fault.

What To Do

1. 1

   Check gas supply.

2. 2

   Reset once.

3. 3

   If A01 returns, call an engineer.

Likely Cause

Flame established then lost — gas pressure or ionisation fault.

What To Do

1. 1

   One reset.

2. 2

   Persistent fault needs engineer.

3. 3

   Call us.

Likely Cause

Primary circuit overheated.

What To Do

1. 1

   Cool fully.

2. 2

   Open all valves.

3. 3

   Reoccurrence requires urgent engineer.

Likely Cause

System pressure has dropped below minimum.

What To Do

1. 1

   Re-pressurise to 1.0–1.5 bar.

2. 2

   Reset.

3. 3

   Repeated loss = leak — call us.

Likely Cause

Fan speed fault.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Fan inspection required.

3. 3

   Call an engineer.

Likely Cause

Flow NTC thermistor failure.

What To Do

1. 1

   One reset.

2. 2

   Sensor replacement required.

3. 3

   Book an engineer.

Likely Cause

Failed ignition — gas supply, electrode, or PCB fault.

What To Do

1. 1

   Check gas supply.

2. 2

   Reset once.

3. 3

   If E10 returns, call an engineer.

Likely Cause

Primary circuit overheated.

What To Do

1. 1

   Cool fully before resetting.

2. 2

   Open all valves.

3. 3

   Engineer required if reoccurs.

Likely Cause

System pressure below safe level.

What To Do

1. 1

   Re-pressurise to 1.0–1.5 bar.

2. 2

   Reset.

3. 3

   Repeated loss = leak — call us.

Likely Cause

Flow NTC thermistor failure.

What To Do

1. 1

   One reset.

2. 2

   Sensor replacement required.

3. 3

   Book an engineer.

Likely Cause

Fan speed outside acceptable range.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Fan inspection required.

3. 3

   Call an engineer.

Likely Cause

Flame not established within the safety time window — gas supply, electrode, gas valve, or burner controller fault.

What To Do

1. 1

   Check other gas appliances to confirm gas supply.

2. 2

   Reset once only.

3. 3

   If L1 returns, call a Gas Safe Remeha engineer — do not keep resetting.

Likely Cause

Primary circuit temperature exceeded the high-limit safety thermostat — pump failure, low flow rate, or blocked heat exchanger.

What To Do

1. 1

   Allow boiler to cool fully (30–60 min) before resetting.

2. 2

   Open all TRVs and zone valves.

3. 3

   If fault reoccurs, call a Gas Safe engineer urgently — do not keep resetting.

Likely Cause

Return NTC sensor open circuit or short circuit — wiring harness or sensor failure.

What To Do

1. 1

   One reset.

2. 2

   If fault persists, sensor and wiring inspection required.

3. 3

   Call a Remeha engineer.

Likely Cause

Flow NTC thermistor open circuit or short circuit — sensor or wiring fault.

What To Do

1. 1

   One reset.

2. 2

   Sensor replacement required if fault persists.

3. 3

   Book a Remeha engineer.

Likely Cause

Flue temperature sensor reading out of range — sensor damage, wiring fault, or flue blockage.

What To Do

1. 1

   Allow boiler to cool.

2. 2

   Check flue for obstructions.

3. 3

   One reset. If fault persists, sensor or flue inspection required — call an engineer.

Likely Cause

Flame detected then lost mid-operation — gas supply drop, ionisation probe fault, or draught issue.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Check gas supply to other appliances.

3. 3

   Call a Gas Safe Remeha engineer — ionisation probe and gas valve inspection required.

Likely Cause

Boiler exceeded safe temperature limit — check pump, water flow rate, and system pressure.

What To Do

1. 1

   Allow boiler to cool fully before resetting.

2. 2

   Ensure all TRVs and zone valves are open.

3. 3

   Call a Gas Safe engineer if fault reoccurs.

Likely Cause

System pressure below minimum operating threshold — repressurise and check for leaks.

What To Do

1. 1

   Locate the filling loop and repressurise to 1.0–1.5 bar.

2. 2

   Reset.

3. 3

   If pressure drops again within days, a leak or failed PRV is present — call us.

Likely Cause

Pressure sensor signal out of range — sensor failure or wiring fault.

What To Do

1. 1

   One reset.

2. 2

   If fault persists, pressure sensor inspection or replacement needed.

3. 3

   Call an engineer.

Likely Cause

Fan not reaching target speed or not responding — fan motor or speed sensor fault.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Fan inspection required.

3. 3

   Call a Remeha engineer.

Likely Cause

Flame not established after repeated ignition attempts — gas valve, ignition electrode, or ionisation probe fault.

What To Do

1. 1

   Check gas supply.

2. 2

   One reset attempt only.

3. 3

   Call a Gas Safe engineer immediately — do not keep resetting.

Likely Cause

eBUS communication error between boiler and external controller (iSense, neoStat, OpenTherm thermostat).

What To Do

1. 1

   Check external controls are powered and connected.

2. 2

   Inspect eBUS wiring at both ends.

3. 3

   One reset. If fault persists, call an engineer.

Likely Cause

Fan speed fault detected — fan motor wear or PCB fault.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Fan inspection required.

3. 3

   Call a Remeha engineer.

Likely Cause

Pressure below minimum threshold.

What To Do

1. 1

   Re-pressurise via filling loop to 1.0–1.5 bar.

2. 2

   Reset.

3. 3

   Repeated pressure loss indicates a leak — call us.

Likely Cause

Boiler is running an automatic high-temperature DHW hygiene programme — normal scheduled function.

What To Do

1. 1

   This is normal — allow the cycle to complete.

2. 2

   If this appears unexpectedly and repeatedly, check DHW settings.

3. 3

   Call an engineer if concerned.

Likely Cause

Annual service interval reached — service required to maintain efficiency and warranty.

What To Do

1. 1

   Book an annual Remeha service with a Gas Safe engineer.

2. 2

   Do not ignore — this maintains your manufacturer warranty.

3. 3

   Reset service counter after completed service.

Likely Cause

Oil burner failed to ignite — oil supply, electrodes, or burner fault.

What To Do

1. 1

   Check oil tank level (minimum 10%).

2. 2

   Reset at burner reset button.

3. 3

   If fault recurs, call an oil engineer — do not reset repeatedly.

Likely Cause

Boiler overheated — pump, flow, or thermostat fault.

What To Do

1. 1

   Cool fully.

2. 2

   Open all valves.

3. 3

   Engineer required if reoccurs.

Likely Cause

Oil flame lost mid-cycle — contaminated oil, nozzle wear, or photocell fault.

What To Do

1. 1

   Check oil quality and filter.

2. 2

   One reset.

3. 3

   Persistent fault needs oil engineer — nozzle or photocell.

Likely Cause

Ignition failure — gas supply, electrode, or controller fault.

What To Do

1. 1

   Check gas supply.

2. 2

   Reset once.

3. 3

   If EA returns, call an engineer.

Likely Cause

Boiler overheated — pump or low flow.

What To Do

1. 1

   Cool fully.

2. 2

   Open all valves.

3. 3

   Reoccurrence needs engineer.

Likely Cause

Flame lost during operation.

What To Do

1. 1

   One reset.

2. 2

   If recurs, engineer required.

3. 3

   Call us.

Likely Cause

System pressure below operating range.

What To Do

1. 1

   Re-pressurise to 1.0–1.5 bar.

2. 2

   Reset.

3. 3

   Repeated loss = leak — call us.

Likely Cause

Ignition failure — gas or electrode fault.

What To Do

1. 1

   Check gas supply.

2. 2

   One reset.

3. 3

   If returns, call engineer.

Likely Cause

Flame established then lost.

What To Do

1. 1

   One reset.

2. 2

   Engineer required for persistent fault.

3. 3

   Call us.

Likely Cause

System pressure below minimum.

What To Do

1. 1

   Re-pressurise to 1.0–1.5 bar.

2. 2

   Reset.

3. 3

   Repeated loss = leak.

Likely Cause

Pressure sensor reading incorrect or failed.

What To Do

1. 1

   One reset.

2. 2

   Sensor replacement if fault persists.

3. 3

   Book an engineer.

Likely Cause

Boiler failed to ignite.

What To Do

1. 1

   Check gas supply.

2. 2

   Reset once.

3. 3

   If 501 returns, call engineer.

Likely Cause

Primary circuit overheated.

What To Do

1. 1

   Cool fully.

2. 2

   Open all valves.

3. 3

   Engineer if reoccurs.

Likely Cause

System pressure has dropped.

What To Do

1. 1

   Re-pressurise to 1.0–1.5 bar.

2. 2

   Reset.

3. 3

   Repeated loss = leak.

Likely Cause

Fan speed fault detected.

What To Do

1. 1

   Do not reset repeatedly.

2. 2

   Fan inspection required.

3. 3

   Call an engineer.

Likely Cause

Failed ignition — gas supply or electrode fault.

What To Do

1. 1

   Check gas supply.

2. 2

   Reset once.

3. 3

   If F1 returns, call engineer.

Likely Cause

Flame lost during operation.

What To Do

1. 1

   One reset.

2. 2

   Engineer required if recurs.

3. 3

   Call us.

Likely Cause

System pressure dropped below minimum.

What To Do

1. 1

   Re-pressurise to 1.0–1.5 bar.

2. 2

   Reset.

3. 3

   Repeated = leak.

Likely Cause

Pressure sensor failed or giving incorrect reading.

What To Do

1. 1

   One reset.

2. 2

   Sensor replacement if fault persists.

3. 3

   Book an engineer.

Likely Cause

Failed ignition — gas or electrode fault.

What To Do

1. 1

   Check gas supply.

2. 2

   Reset once.

3. 3

   If E1 returns, call engineer.

Likely Cause

System pressure below minimum.

What To Do

1. 1

   Re-pressurise to 1.0–1.5 bar.

2. 2

   Reset.

3. 3

   Repeated = leak.

Likely Cause

Boiler overheated.

What To Do

1. 1

   Cool fully.

2. 2

   Open all valves.

3. 3

   Engineer required if reoccurs.