Glow-worm F64 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F64 fault code mean?
The F64 code on a Glow-Worm boiler signals that the main printed circuit board (PCB) has detected an internal electronic fault and has shut the boiler down as a safety precaution. The PCB acts as the boiler's central controller, coordinating ignition, heat output, pump operation, and safety monitoring — so when it reports an error, the boiler locks out entirely, leaving you with no heating or hot water. F64 sits within a group of related PCB-level codes (F61 through F67) that all point to electronic control faults of varying types: F61 and F62 typically relate to internal board communication errors, F63 to a driver circuit issue, F64 to a general board-level fault, F65 to a temperature-monitoring circuit error, and F67 to an ignition-circuit fault on the board. If your display is showing any of these codes, the diagnostic approach described here applies broadly to all of them.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty or failed PCB Common
The control board itself may have reached the end of its serviceable life, suffered component-level failure, or developed an internal short circuit. This is the most direct cause of an F64 lockout and becomes increasingly likely on boilers over eight to ten years old.
- Loose or corroded wiring connections Common
Vibration from the pump and fan, combined with repeated heat-cycling over the years, can gradually work wiring connectors loose on the PCB or elsewhere in the boiler's electrical harness. A poor connection can mimic a board fault and trigger F64 without the PCB itself being defective.
- Moisture or water ingress Sometimes
Condensation inside the boiler casing, a nearby leak, or even a blocked condensate system can allow moisture to reach the PCB or ignition leads. Water on the board can cause short circuits, corrosion, and unpredictable fault codes including F64.
- Power surge or mains voltage spike Sometimes
An electrical surge from the mains supply can damage sensitive PCB components in one event, sometimes also affecting the fan or ignition system at the same time. This is more likely in properties with older wiring or no surge protection.
- Faulty sensor sending bad data to the board Rare
Occasionally, a rogue signal from a temperature sensor or another peripheral component can cause the PCB to report an internal error when the board itself is actually fine. Engineers routinely check sensors before condemning the board outright.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button on your Glow-Worm boiler for approximately one second, then release. Allow the boiler two to three minutes to attempt a restart. A single reset is a reasonable first step; if the boiler fires up and runs normally, monitor it over the next few hours. Do not reset the boiler more than two or three times in total — repeated resets on a PCB fault can cause further damage.
- Check your mains power and consumer unit DIY safe
Confirm the boiler's fused spur or circuit breaker has not tripped. A tripped breaker or a blown fuse at the spur can sometimes look like a boiler fault. Reset the breaker if it has tripped and try the boiler again. If the breaker trips again immediately, stop and call an electrician before proceeding.
- If the fault code returns, call a Gas Safe registered engineer Gas Safe engineer
A persistent F64 requires professional diagnosis. The engineer will use a multimeter to test the electrical resistance and continuity of the PCB connections, wiring harness, and associated components such as sensors and the ignition lead. This methodical approach identifies whether the fault lies with the board itself or with a peripheral component sending bad signals to it.
- Engineer inspection of wiring and connections Gas Safe engineer
Before replacing the PCB, a competent engineer will inspect all wiring connectors for looseness, heat damage, or corrosion and reseat any that are suspect. In a number of F64 cases, reseating a connector clears the fault entirely, saving the cost of a new board.
- Repair or replacement of the faulty component Gas Safe engineer
If a sensor, ignition lead, or other component is identified as the root cause, the engineer will replace that part — generally a far more economical outcome than a full PCB replacement. If the board itself has failed, a like-for-like replacement PCB will be sourced and fitted, with all connections checked afterwards before the boiler is test-fired.
Parts you may need
- Replacement PCB (main control board) · from £280
- Ignition lead / electrode assembly · from £35
- NTC temperature sensor · from £25
- Wiring harness / loom · from £60
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £300–£600, depending on the underlying cause.
Frequently asked questions
Can I fix a Glow-Worm F64 fault myself?
No. Because F64 is a PCB-level electronic fault, the diagnosis and any repair work must be carried out by a Gas Safe registered engineer. The only homeowner action worth trying is a single boiler reset. Working on the board, wiring, or any internal component yourself is unsafe and will void your warranty or boiler cover policy.
How much does it cost to fix an F64 fault on a Glow-Worm boiler?
Most people with an F64 fault pay between £300 and £600 all-in, which covers the engineer's diagnostic visit and the cost of parts and labour. If the fault turns out to be a loose connection or a faulty sensor rather than the board itself, you could pay considerably less. A full PCB replacement sits at the higher end — typically £450 to £600 including labour. In very rare cases where a premium or obsolete PCB is needed, costs can exceed this, so it is worth getting a written quote before authorising the work.
Is it worth replacing the PCB on an older Glow-Worm boiler?
If your boiler is more than ten years old and has been unreliable, replacing only the PCB may not represent good value — a new A-rated boiler will be more efficient and is unlikely to have further control-board issues for many years. Discuss the boiler's overall condition with your engineer before committing to a PCB replacement. For a boiler under eight years old in otherwise good condition, a PCB repair or replacement is usually well worthwhile.
What is the difference between F61, F62, F63, F64, F65, and F67 on a Glow-Worm boiler?
All of these codes sit in the same PCB fault group and indicate different types of internal electronic error. F61 and F62 are typically linked to communication errors within the board's circuitry; F63 points to a driver circuit issue; F64 is a general board-level electronic fault; F65 relates to the board's internal temperature-monitoring circuit; and F67 suggests a fault with the ignition circuit as seen by the PCB. In practice, the diagnostic and repair process is similar for all of them — a Gas Safe engineer will test the board and associated components to identify exactly what has failed.