How to Fix Viessmann F4 Fault Code (Flame Loss) UK
Viessmann F4 means flame loss. UK diagnosis, parts to order, reset sequence.
What F4 means
Viessmann F4 indicates flame loss. The boiler will lock out and refuse to fire until the fault is cleared.
Do not simply reset — F4 is stored in the Viessmann error log and will re-trigger within minutes if the underlying part has failed.
Diagnostic sequence
Enter service mode on the Viessmann boiler and read the flow/return/DHW temperatures, fan PWM, ionisation current and pump speed.
Cross-reference against the Viessmann technical manual — F4 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 Viessmann test procedure.
Parts to have on the van
Carry a set of NTC sensors, an ignition electrode kit and a fan for Viessmann — these three parts clear >70% of F4 call-outs.
Recommission
After the fix, run the Viessmann 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 F4 without changing parts?
- Only if the fault has cleared (e.g. low pressure repressurised). Persistent F4 means a hardware failure.
- Which is the most common cause of Viessmann F4?
- On Viessmann boilers, F4 most often points to the sensor or actuator matching its meaning (flame loss); 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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