Glow-worm F.65 fault codewhat it means and how to fix it
F.65 means the main board itself has got too hot — from something outside it, Glow-worm say first, or because it is failing. The boiler stops to protect its electronics.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.65
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What F.65 means
F.65 on the Energy, Ultimate, Easicom and Compact is "Fault: electronics temperature too high", and Glow-worm's two causes put the outside world first: "electronics overheating due to external influences, electronics defective". The Compact manual's first measure is "check the external heat effects on the electronics" and only then "replace the PCB". The board has a temperature sensor of its own and stops the boiler when it reads more than the components are rated for.
External influences in Glow-worm's sense are a boiler boxed in a cupboard with no ventilation gap, a casing left off so the board sits in the heat rising from the exchanger, a hot water leak dripping onto the electronics compartment, or a room that itself runs hot — an airing cupboard with the boiler above the cylinder is a common setting. The two-digit Ultracom2, Betacom 24a/28a and Betacom2 have F65 as "excessive main board temperature" and share the F61-to-F67 measures, ending with a reset that will work once the board has cooled. There is no F65 in the Ultracom, Flexicom or CI tables.
The householder's contribution is to notice the surroundings: if the code appears on hot afternoons or after a long hot water draw and clears when the boiler cools, the environment is the cause and the engineer needs to know it.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Boiler enclosed without the clearances Glow-worm require | The first of Glow-worm's "external influences". A cupboard built tight to the casing, or shelving that blocks the ventilation gaps the installation manual specifies, lets heat build round the electronics. | Common |
| Heat from a leak or an exchanger running hot | A weep from a joint above the electronics compartment, or a heat exchanger scaled so that the casing runs hotter than designed, warms the board from inside the boiler. | Occasional |
| Board component failing | "Electronics defective." A component drawing more current than it should heats the board from within; the code returns even in a cool, well-ventilated setting and the board is replaced. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£380
Time on site
1 hour
A ventilation or leak problem corrected £80–£150 plus any joinery. A main PCB £250–£380 fitted.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Glow-worm main PCB
£150–£320Authorised Glow-worm parts distributor
Price checked 28 September 2026
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Related Glow-worm fault codes
- F.70Get it checkedF.70 means the boiler does not know which model it is — the Device Specific Number is missing, wrong or does not match the coding resistor, usually after a board or display has been changed.Related: electronics and communication
- F.49Get it checkedF.49 is an eBUS fault — the control bus voltage has dropped through a short, an overload or two supplies wired with opposite polarity. It points at the wiring to your controls.Related: electronics and communication
- F.63Get it checkedF.63 is an EEPROM fault — the memory on the main board has failed its check. Glow-worm give one cause, "electronics defective", and one measure, replace the PCB. Nothing else clears it.Related: electronics and communication
- F15Get it checkedF15 is the eBUS voltage on an Ultracom, Flexicom or Ultrapower, and the gas valve stepper motor on a cxi, CI or CI Plus. Know which boiler you have before anyone orders a part.Related: electronics and communication
- F.64Get it checkedF.64 is the board's analogue-to-digital converter reporting a reading it cannot make sense of — Glow-worm blame a shorted flow or return sensor first and the electronics second.Related: electronics and communication
- F12Get it checkedF12 on a cxi, sxi, CI or Xtramax is the link between the main board and the display board. F.12 on an Energy7 system boiler is a cylinder sensor short. Different generations, both engineer's codes.Related: electronics and communication
Own a Glow-worm boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.