How to Fix Ferroli F34 Fault Code (Supply Voltage Low) UK
Ferroli F34 means supply voltage low. UK diagnosis, parts to order, reset sequence.
What F34 means
Ferroli F34 indicates supply voltage low. The boiler will lock out and refuse to fire until the fault is cleared.
Do not simply reset — F34 is stored in the Ferroli error log and will re-trigger within minutes if the underlying part has failed.
Diagnostic sequence
Enter service mode on the Ferroli boiler and read the flow/return/DHW temperatures, fan PWM, ionisation current and pump speed.
Cross-reference against the Ferroli technical manual — F34 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 Ferroli test procedure.
Parts to have on the van
Carry a set of NTC sensors, an ignition electrode kit and a fan for Ferroli — these three parts clear >70% of F34 call-outs.
Recommission
After the fix, run the Ferroli 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 F34 without changing parts?
- Only if the fault has cleared (e.g. low pressure repressurised). Persistent F34 means a hardware failure.
- Which is the most common cause of Ferroli F34?
- On Ferroli boilers, F34 most often points to the sensor or actuator matching its meaning (supply voltage low); 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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