How to Fix Glow-worm F65 Fault Code (Pcb Overheat) UK
Glow-worm F65 means PCB overheat. UK diagnosis, parts to order, reset sequence.
What F65 means
Glow-worm F65 indicates PCB overheat. The boiler will lock out and refuse to fire until the fault is cleared.
Do not simply reset — F65 is stored in the Glow-worm error log and will re-trigger within minutes if the underlying part has failed.
Diagnostic sequence
Enter service mode on the Glow-worm boiler and read the flow/return/DHW temperatures, fan PWM, ionisation current and pump speed.
Cross-reference against the Glow-worm technical manual — F65 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 Glow-worm test procedure.
Parts to have on the van
Carry a set of NTC sensors, an ignition electrode kit and a fan for Glow-worm — these three parts clear >70% of F65 call-outs.
Recommission
After the fix, run the Glow-worm 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 F65 without changing parts?
- Only if the fault has cleared (e.g. low pressure repressurised). Persistent F65 means a hardware failure.
- Which is the most common cause of Glow-worm F65?
- On Glow-worm boilers, F65 most often points to the sensor or actuator matching its meaning (PCB overheat); 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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