How to Fix Ferroli F35 Fault Code (Supply Frequency Low) UK
Ferroli F35 means supply frequency low. UK diagnosis, parts to order, reset sequence.
What F35 means
Ferroli F35 indicates supply frequency low. The boiler will lock out and refuse to fire until the fault is cleared.
Do not simply reset — F35 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 — F35 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 F35 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 F35 without changing parts?
- Only if the fault has cleared (e.g. low pressure repressurised). Persistent F35 means a hardware failure.
- Which is the most common cause of Ferroli F35?
- On Ferroli boilers, F35 most often points to the sensor or actuator matching its meaning (supply frequency 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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