Glow-worm F12 fault codewhat it means and how to fix it
F12 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.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F12
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.
Which display are you looking at?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
F12 or 12 with a flashing red light on a cxi, sxi, SI, CI, CI Plus, Xtramax or Xtrafast
The connection between the display board and the main board, or one of the boards.
Book a Gas Safe registered engineer and mention any flickering or blank display beforehand.
F.12 with a full stop on an Energy7 system or regular boiler
Cylinder temperature sensor short circuit — the same fault as F.13.
Book a Gas Safe registered engineer for the cylinder thermistor and its cable.
What F12 means
On the Hepworth-era boilers — the cxi, sxi, SI, CI and CI Plus — Glow-worm's trade tables list F12 as "connection fault between main PCB and display PCB"; the Xtramax HE table has "user interface connection fault", the Xtrafast simply "user interface fault", and the 24CI/18SI manual gives 12 as "user interface or distant control panel fault". All of them describe the same thing: the ribbon or lead that carries the display board's conversation with the main board has stopped working, either because a plug has loosened, the lead has failed, or one of the two boards has.
The display may go blank, flicker, or show F12 alongside F11 and F18, which are the neighbouring codes for the same link and the interface itself. The Ultracom hxi, cxi and sxi and the Flexicom carry no F12; their board codes are F11 and F13. Nor do the two-digit Ultracom2 and Betacom boards.
On the current F.xx ranges the characters recur with an unrelated meaning: the Energy7 installation manual lists "F.12 and F.91 — short circuit: cylinder temperature sensor" beside F.13, which applies only to a system or regular boiler wired to a hot water cylinder, and the Energy 35 Store manual has F.12 as "cylinder charging sensor short circuit" on its built-in store; both are covered on the F.13 page. So a reader has to know which display they are looking at — a two-digit number with a flashing red light on a boiler from the early 2000s, or F.12 with a full stop on a recent system boiler — before either explanation applies. Neither generation gives the householder a step.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Display-to-main-board lead or plug — cxi, sxi, SI, CI, Xtramax | Glow-worm's "connection fault between main PCB and display PCB". The lead runs through the hinged fascia and its plugs work loose over years of the panel being opened; the 24CI manual has the fascia unclipped and hinged forward for every board job. | Common |
| Display board or main board failed | If the lead and plugs prove good the fault is in one of the boards at either end. The engineer proves the cheaper display board first. | Occasional |
| Cylinder sensor short — Energy7 system and regular boilers | The current-range meaning. "NTC sensor defective, short circuit in the cable harness"; the cylinder thermistor and its long cable are metered and the failed part replaced. See the F.13 page. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£300
Time on site
1 hour
A lead reseated or replaced £80–£150. A display or main board on an older range £200–£300 where still available. A cylinder thermistor on an Energy7 £80–£160.
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
- 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
- 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 support portal
- Manufacturer technical and service documentation
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.