Glow-worm F18 fault codewhat it means and how to fix it
F18 on a cxi, sxi, SI, CI or CI Plus is a fault in the user interface — the display and button board itself, where F12 is its link to the main board.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F18
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 F18 means
The Hepworth-era Glow-worm boilers report their control panel in three codes. F11 is "main board connection fault" and F12 the "connection fault between main PCB and display PCB" — the lead between the two boards. F18 is the third: "user interface fault" in the trade tables for the cxi, sxi, SI, CI and CI Plus, and "local user interface fault" in the 24CI/18SI manual, meaning the display and button board itself has failed its checks rather than lost contact with the main board. The same manual also lists 20 as "user interface not compatible with boiler", which is the code for a replacement panel of the wrong type.
In practice an F18 panel shows odd segments, refuses buttons, or shows the code intermittently as the board warms; the 24CI manual's replacement procedure has the fascia unclipped and hinged forward, the connections disconnected and the board swapped, and it is not a householder's job.
The Ultracom, Flexicom and Ultrapower have no F18, the two-digit Ultracom2 and Betacom boards report a display problem as Err, and the current F.xx ranges have no interface code at all. Whether a twenty-year-old user interface board is still available is the question the engineer will answer before ordering, and it is fair to ask about the cost of the alternative at the same time.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| User interface board failed | Glow-worm's only cause. The display and button board has failed its self-check, often after years of heat cycling behind the fascia. Replaced as a board. | Common |
| Moisture or a knocked connector on the panel | A drip from a joint above the fascia, or a connector disturbed when the panel was hinged open, can fail the board's check without the board being dead. Checked before ordering. | Occasional |
| Wrong replacement panel | The 24CI manual lists a separate code, 20, for a user interface not compatible with the boiler; a replacement of the wrong type can also show as a general interface fault. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£150–£300
Time on site
1 hour
A user interface board on a CI-era boiler £150–£300 fitted, where still available.
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 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.