Glow-worm F.97 fault codewhat it means and how to fix it
F.97 means the main board has failed its own self-test. Glow-worm give one cause and one measure — the PCB is defective and is replaced. It is on the current ranges only.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.97
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.97 means
F.97 is the last code in the Energy7, Ultimate3 and Compact tables and the shortest: "Fault: main PCB self-test failed — main PCB defective", and in the Compact manual "PCB defective — replace the PCB". The board runs a check of its own safety-relevant circuits at power-up and at intervals while running; if a check fails it stops the boiler and reports itself.
Unlike F.63, which is the board's memory, or F.64 and F.67, which are inputs the board cannot make sense of, F.97 is the board declaring that its own logic cannot be trusted, and Glow-worm offer no alternative cause to rule out first. It does not appear in the two-digit Ultracom2 or Betacom tables or in any older Glow-worm list — those boards had no equivalent self-test code, and a failed board on them tends to show as F13 or as a dead display.
An engineer will still check the supply and the earth to the board before replacing it, because a poor mains supply or a fault on the harness can occasionally fail a self-test on a board that is otherwise sound, and because the new board needs those to be right as well. The new board is then configured with the Device Specific Number from the nameplate. There is no householder step; a boiler showing F.97 is off until the board is changed.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Main board self-test failed | Glow-worm's only cause. A safety-relevant circuit on the PCB no longer passes its own check, and the board refuses to run. Replaced as a unit. | Common |
| Supply or earth problem failing the test | Not in Glow-worm's list, but checked by engineers before the board is condemned: a poor earth or an unstable mains supply can fail a self-test on a sound board and will fail a new one the same way. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£250–£380
Time on site
1 hour
A main PCB fitted and configured £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 main PCB
£150–£320Authorised Glow-worm parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
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
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.