How to Fix Vaillant F67 Fault Code (Unusual Flame Signal) UK
Vaillant F67 means unusual flame signal. UK diagnosis, parts to order, reset sequence.
What F67 means
Vaillant F67 indicates unusual flame signal. The boiler will lock out and refuse to fire until the fault is cleared.
Do not simply reset — F67 is stored in the Vaillant error log and will re-trigger within minutes if the underlying part has failed.
Diagnostic sequence
Enter service mode on the Vaillant boiler and read the flow/return/DHW temperatures, fan PWM, ionisation current and pump speed.
Cross-reference against the Vaillant technical manual — F67 typically points to two or three candidate parts.
- Check wiring harness continuity to the suspect sensor before condemning it.
- Isolate at the fused spur, discharge and re-inspect connectors.
- Measure resistance / voltage at the PCB plug per Vaillant test procedure.
Parts to have on the van
Carry a set of NTC sensors, an ignition electrode kit and a fan for Vaillant — these three parts clear >70% of F67 call-outs.
Recommission
After the fix, run the Vaillant boiler through a full flow test, gas rate, and combustion analysis. Log to the Benchmark book to preserve manufacturer warranty.
Frequently asked questions
- Can I reset F67 without changing parts?
- Only if the fault has cleared (e.g. low pressure repressurised). Persistent F67 means a hardware failure.
- Which is the most common cause of Vaillant F67?
- On Vaillant boilers, F67 most often points to the sensor or actuator matching its meaning (unusual flame signal); check wiring and connector integrity first.
- Is a Gas Safe engineer required?
- Yes — any works past the boiler case seal require a Gas Safe registered engineer under UK law.
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