How to Fix Vaillant F27 Fault Code (UK)
Vaillant showing F27 — Flame simulation. UK diagnosis, likely spare (PCB / ionisation lead) and step-by-step reset.
What the code means
F27 on a Vaillant indicates flame simulation. The boiler locks out to protect the heat exchanger and, on gas faults, to comply with the Gas Safety (Installation and Use) Regulations. Always reset to confirm the code is live, not historical.
Likely spare
The most common fix is the PCB / ionisation lead. Check the exploded diagram against the GC number on the data plate before ordering — Vaillant revised parts on Vaillant units mid-production.
Fix procedure
Check ionisation lead not shorted. Replace PCB if flame signal present with no call.
Log the fault and reset. If the code returns within 24 hours, escalate to full electrical diagnosis with a manometer and multimeter.
Frequently asked questions
- Can I reset F27 myself?
- You can press the reset button up to 3 times. If F27 returns, the boiler needs a Gas Safe engineer — it is illegal to repair gas components without registration.
- Is the part covered under warranty?
- Vaillant standard warranty is 2–10 years depending on model. Registered installations get the full term. Check the sticker inside the case.
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