Vaillant F28 Fault Code: Ignition Failure — Causes & Same-Day Fix

What does the Vaillant F28 fault code mean?

The Vaillant F28 fault code indicates that your boiler has attempted to ignite the burner but failed to detect a flame within the expected timeframe. When ignition fails, the boiler locks out as a critical safety measure — it will not keep trying to release gas into the combustion chamber without a confirmed flame. You'll have no heating and no hot water until the fault is resolved.

F28 is one of the most serious Vaillant fault codes because it involves the gas and ignition system. Unlike pressure-related codes like F22 which you can sometimes resolve yourself, F28 requires diagnosis by a Gas Safe registered engineer. Attempting to bypass the ignition lockout is dangerous and can lead to gas accumulation in the flue.

Across our years repairing Vaillant boilers in London — from Belgravia to Hampstead — F28 is the ignition fault we see most frequently on the ecoTEC range. The good news is that with correct diagnosis, most F28 faults are fixable on the first visit.

The 5 most common causes of Vaillant F28

1. No gas supply or low gas pressure

The first thing our engineers check is whether gas is reaching the boiler. F28 can be triggered if:

• The gas supply is turned off at the meter or an isolation valve
• The gas meter has credit on a prepayment meter
• Gas pressure is too low for the burner to ignite reliably
• Other gas appliances in the property are working (confirming supply) or not (suggesting a supply issue)

If other gas appliances (hob, fire) are also not working, the issue is with your gas supply, not the boiler — contact your gas supplier. If only the boiler is affected, the fault is boiler-internal.

2. Ignition electrode failure

The ignition electrode creates the spark that lights the gas burner. If the electrode becomes contaminated with carbon build-up, cracked, or fails electrically, no spark is produced and ignition cannot occur. This is one of the most common causes of F28 on Vaillant ecoTEC boilers.

Electrode contamination happens gradually over years of use — carbon deposits from combustion coat the electrode tip until the spark can no longer jump the gap. Our engineers clean or replace the electrode as part of annual servicing to prevent this.

3. Gas valve fault

The gas proportional valve opens to allow gas to flow to the burner when the boiler calls for heat. If the valve is stuck closed, has lost its electrical supply, or has failed internally, gas won't reach the burner and ignition will fail — triggering F28.

Gas valve faults can be caused by:

• Electrical connection failure to the valve
• Internal mechanical seizure (often due to debris or age)
• PCB not sending the open signal correctly
• Valve regulator failure causing insufficient gas flow

Gas valve replacement is a more expensive repair — typically £250–£450 including parts and labour — but is necessary when the valve itself has failed.

4. Flame sensor / ionisation electrode fault

Even if ignition occurs, the flame sensor (ionisation electrode) must detect the flame to confirm successful ignition. If the sensor is contaminated, misaligned, or has failed, it won't detect the flame and the boiler will shut down with F28 — even though the burner actually lit briefly.

This is a subtle fault because the burner does ignite momentarily before the sensor fails to confirm it. Our engineers test the ionisation current to determine whether the sensor is reading correctly. Flame sensor replacement is typically £120–£200 including parts and labour.

5. Flue or ventilation issue

Vaillant boilers monitor flue conditions as part of the ignition safety sequence. If the flue temperature sensor detects abnormal readings, or if there's a blockage in the flue or air intake, the boiler may prevent ignition and display F28 (or a related flue code). This is less common but does occur — particularly on boilers with flues that pass through cold or exposed areas where condensation can freeze in winter.

Flue-related F28 faults require careful inspection of the entire flue run, terminal position, and air intake path. Our engineers carry flue gas analysers to test combustion performance after any flue-related repair.

Why F28 is not a DIY fix

Unlike pressure faults (F20, F22) which you can sometimes resolve by re-pressurising, F28 involves the gas and ignition system. Working on gas components without Gas Safe registration is illegal and dangerous. Specifically:

• Ignition electrode replacement requires removing the burner assembly and resealing the combustion chamber correctly
• Gas valve replacement requires commissioning with a flue gas analyser to set the gas/air ratio correctly
• Flame sensor adjustment requires testing ionisation current with specialist equipment
• PCB diagnosis requires multimeter testing of multiple signal paths

Incorrect diagnosis of F28 often leads to unnecessary parts replacement — generalist engineers may replace the gas valve when the electrode simply needs cleaning, costing you hundreds of pounds more than necessary.

How RARE engineers diagnose F28

Our Vaillant specialists follow a structured diagnostic sequence on every F28 call-out:

1. Verify gas supply at the boiler inlet using a manometer
2. Check ignition electrode for contamination, cracking, and correct gap
3. Test electrode spark voltage and continuity
4. Inspect the flame sensor / ionisation electrode for contamination and alignment
5. Test ionisation current during attempted ignition
6. Verify gas valve electrical connections and operation
7. Check flue gas analyser readings if the boiler does ignite briefly
8. Test PCB outputs to ignition and gas valve if all components test normal

This systematic approach means we fix F28 correctly the first time — and we don't replace expensive components unnecessarily.

What does an F28 repair cost?

Our standard rate is £150/hr + VAT with no call-out charge (minimum 1 hour). Out-of-hours is £195/hr + VAT, bank holidays £275/hr + VAT. Typical repair costs by component:

• Ignition electrode cleaning: included in the first hour's labour
• Ignition electrode replacement: £150–£250 including parts and labour
• Flame sensor replacement: £120–£200 including parts and labour
• Gas valve replacement: £250–£450 including parts, labour, and commissioning
• PCB fault: £350–£600 depending on the model and whether repair or replacement is needed

We always quote transparently before any work begins — no surprise charges.

How to prevent F28

• Annual boiler servicing — we clean the ignition electrode and flame sensor every year to prevent carbon build-up
• Keep the area around your boiler well-ventilated and free from obstructions
• If your flue terminates outside, check it's not blocked by debris, snow, or vegetation
• Consider a PPM maintenance plan — annual service included plus priority emergency response
• If your Vaillant is over 12 years old and F28 is recurring, a new ecoTEC installation may be more economical than repeated repairs

Related Vaillant fault codes

• F22 — Low water pressure. Often confused with F28 but unrelated (pressure, not ignition).
• F75 — No pressure change on pump start. Can occur alongside F28 if the pump isn't running.
• F29 — Flame loss during operation. Related to F28 but the flame goes out after ignition rather than failing to ignite.

For a full list of Vaillant and other brand fault codes, use our Boiler Error Codes lookup tool.

Book a Vaillant engineer

F28 requires a Gas Safe registered engineer — do not attempt to fix it yourself. RARE Plumbing's Vaillant specialists carry ignition electrodes, flame sensors, and gas valves on every van, so most F28 faults are fixed on the first visit. Call 020 7458 4949 or book a Vaillant specialist online.