How to Fix Viessmann F38 Fault Code (High Supply Temp) UK
Viessmann F38 means high supply temp. UK diagnosis, parts to order, reset sequence.
What F38 means
Viessmann F38 indicates high supply temp. The boiler will lock out and refuse to fire until the fault is cleared.
Do not simply reset — F38 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 — F38 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 F38 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 F38 without changing parts?
- Only if the fault has cleared (e.g. low pressure repressurised). Persistent F38 means a hardware failure.
- Which is the most common cause of Viessmann F38?
- On Viessmann boilers, F38 most often points to the sensor or actuator matching its meaning (high supply temp); 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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