Vaillant F.75 Fault Code Explained
What the Vaillant F.75 fault means, the five most common causes, and how RARE fixes it across London.
Fault Code
F.75
Meaning
No pressure change detected on pump start
Severity
Boiler lockout — no heating or hot water
The Vaillant F.75 fault code indicates that when the boiler attempted to start, the pump failed to generate the expected pressure differential within the primary circuit. The boiler's control board detected no pressure change on pump activation and shut down the boiler as a safety measure.
F.75 is one of the most common fault codes on Vaillant ecoTEC boilers — which are widely installed across prime London properties in Kensington, Chelsea, Marylebone and across the SW and W postcode areas. The fault has five possible causes, and correctly identifying which one applies requires systematic testing rather than immediately replacing the pump.
When our Gas Safe registered engineers attend an F.75 call-out, the first task is to distinguish between a mechanical pump failure (the most common cause), a sensor fault, and the less common hydraulic causes. The distinction matters because replacing an expensive component unnecessarily wastes time and money.
The 5 Causes of Vaillant F.75
Failed Pump
The most common cause. If the pump is not running — due to seizure, electrical fault or blocked impeller — the pressure sensor sees no pressure differential and throws F.75. The pump may still hum (motor energised but impeller seized) or be completely silent.
Faulty Pressure Sensor
The hydraulic pressure switch or electronic pressure sensor itself may have failed. This is the second most likely cause — particularly on boilers that have previously had sludge issues, as contamination can damage the sensor membrane.
Air Lock in Heat Exchanger
Air trapped in the primary circuit prevents the pump from building pressure. This can occur after a system drain, a repair or if the boiler was allowed to run dry. Venting the air correctly — not just topping up pressure — is the solution.
Sludge Blockage
Magnetite sludge can block the inlet to the pressure sensor or restrict circulation sufficiently to prevent pressure build-up. London's hard water compounds this — scale and sludge together block narrow passages in the heat exchanger.
Wiring or PCB Fault
Less commonly, the fault lies in the signal path between the pressure sensor and the PCB, or in the PCB itself. This is typically identified after pump and sensor replacement have been eliminated.
Diagnostic Process
RARE engineers follow a structured diagnostic sequence for every F.75 call-out:
- Check system pressure — below 0.5 bar can trigger F.75 independent of the pump
- Attempt manual pump release via impeller bleed screw
- Test pump motor with multimeter — confirm electrical supply and continuity
- Test hydraulic pressure switch/sensor directly
- Bleed primary circuit to eliminate air lock
- Inspect for sludge blockage at pump inlet and sensor port
- Test PCB output to pump if all components test normal
Related Vaillant Fault Codes
Vaillant F.75 on your boiler?
RARE carry Vaillant pumps, pressure sensors and parts on our service vans across London. Same-day attendance available. Gas Safe registered engineers.
Frequently Asked Questions
Can I fix Vaillant F.75 myself?+
You can attempt a simple pump impeller release (inserting a flathead screwdriver into the pump head bleed screw to manually turn the impeller) if the pump is seized but the motor is intact. However, gas boiler work must be carried out by a Gas Safe registered engineer. Do not attempt to replace the pump, sensor or PCB yourself.
How long will an F.75 repair take?+
Pump replacement typically takes 60–90 minutes. Sensor replacement is usually faster — 30–45 minutes. Where sludge blockage is the cause, additional flushing work may be required. Our engineers carry Vaillant pumps and pressure sensors in their vans.
Will my Vaillant warranty cover an F.75 fault?+
If the boiler is within warranty and has been serviced annually by a Gas Safe engineer, the pump and sensor are generally covered. If annual servicing records cannot be produced, Vaillant may reject the warranty claim — even if the boiler is within the warranty period.
How much does it cost to fix a Vaillant F.75 fault?+
Pump replacement costs typically £200–£350 including parts and labour. Pressure sensor replacement is usually £120–£200. Where the PCB is at fault, costs are higher — our engineers will quote transparently before any work begins.
Why does my Vaillant keep showing F.75 after repair?+
If F.75 returns after pump replacement, the pressure sensor is likely also damaged (common where sludge was involved). If it returns after both components have been replaced, the PCB or wiring loom may be at fault, or the system has a hydraulic issue that's masking as an F.75.
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