How to Diagnose a Boiler Fault Code (UK, 2026)
Decode UK boiler fault codes: Worcester EA229, Vaillant F75, Baxi E133, Ideal L2, Glow-worm F22. Root cause and the spare you need.
Worcester BoschVaillantBaxiIdealGlow-wormPotterton
The four categories of fault code
Every modern condensing boiler fault falls into one of four buckets: ignition (no gas / no spark), flame (rectification failure), sensor (NTC / flow / return), or safety (overheat / pressure / flue).
Top UK fault codes and the spare to buy
- Worcester EA229 — no flame after ignition. 90 % gas valve or ignition lead (part 87161424040).
- Vaillant F75 — pump/pressure sensor. Fit low-water-pressure sensor 0020219476 first, pump 0020221215 if unchanged.
- Baxi E133 — flame loss. Check condensate trap, then gas valve 720078201.
- Ideal Logic L2 — ignition lock-out. Ignition lead 175593, electrode 175584.
- Glow-worm F22 — low pressure. Auto air vent + expansion vessel 0020014173.
When it isn't the boiler
Repeated F22 / low pressure faults are almost never the PRV. It's a failed expansion vessel losing charge, or a leaking rad valve. Pump down, recharge the vessel to 1 bar, refill to 1.2 bar cold. If pressure climbs above 2.5 bar hot, the vessel is done.
Frequently asked questions
- Can I clear a fault code by resetting?
- A reset clears a soft lock-out. If the same code returns within one heat demand, it is a hardware fault and the spare must be replaced.
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