Glow-worm F.64 fault codewhat it means and how to fix it
F.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.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.64
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What F.64 means
F.64 sits in the electronics block of Glow-worm's table but its first cause is a sensor. The Energy7 and Ultimate3 manuals give it as "Fault: electronics / sensor / analogue-to-digital converter" with the causes "flow or return NTC short circuited, electronics defective", and the Compact manual orders its measures the same way: check the flow temperature sensor, check the return temperature sensor, then the PCB.
The reason is that a thermistor shorted hard enough drives the converter's input beyond the range it can digitise, and the board reports the converter rather than the sensor that overloaded it. On the two-digit Ultracom2, Betacom 24a/28a and Betacom2 the code is F64, "rapid fluctuation of return or flow heating sensor", which describes the symptom from the other side — a reading jumping too fast to be real — and shares the F61-to-F67 measures of checking the board's connections, the board, the DSN and a reset. There is no equivalent in the Ultracom, Flexicom or CI tables.
An engineer meters both NTCs before condemning the board, because a thermistor is a small part and a board is not, and F.64 appearing alongside F.10 or F.11 in the fault history all but confirms the sensor. No Glow-worm user guide offers a householder step.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Flow or return thermistor shorted | Glow-worm's first cause. A hard short on either NTC overloads the converter input and the board raises F.64 instead of, or as well as, F.10 or F.11. | Common |
| Harness short or water in a sensor plug | The same overload from the wiring rather than the sensor. A leak from the heat exchanger onto the sensor plugs below is the classic route. | Occasional |
| Converter on the main board failed | "Electronics defective." Confirmed only when both sensors and their wiring meter correctly; the board is then replaced and configured. | 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 thermistor £80–£160 fitted if that is all it is. A main PCB £250–£380.
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 flow or return temperature sensor (clip-on NTC)
£12–£35Authorised Glow-worm parts distributor
Price checked 28 September 2026
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
- 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
- F.65Get it checkedF.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.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.