Glow-worm F90 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F90 fault code mean?
The F90 code on a Glow-worm boiler signals a communication breakdown between the boiler's main printed circuit board (PCB) and the user interface or display panel. Because Glow-worm is part of the Vaillant Group and shares much of the same engineering platform, this fault mirrors the Vaillant F.90 code. The boiler enters a hard lockout — it will not fire or heat water until the fault is resolved. A note on identification: some older Glow-worm displays can show 'F9 0' in a way that reads as F90, so if your boiler is an older model, check your manual to confirm whether it is displaying F9 (a separate fault) or a genuine F90. On modern Glow-worm boilers such as the Energy, Ultimate, and Flexicom ranges, F90 consistently relates to PCB-to-interface communication loss.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Loose or corroded wiring between PCB and display panel Common
Vibration from the boiler's normal operation can gradually loosen the wiring harness connectors that link the PCB to the user interface. Corrosion from condensation inside the casing can also degrade contact quality, breaking the data signal between the two components.
- Faulty user interface or display panel Common
The display unit itself can develop internal faults — a cracked ribbon connector, a failed component on the panel's own circuit board, or physical damage from moisture. Because the display is cheaper than the PCB, engineers typically test and rule this out first before assuming the PCB is at fault.
- Damaged or failed PCB Sometimes
The PCB can fail due to general wear as the boiler ages, moisture ingress from a nearby leak, or damage from a power surge. A failed PCB will disrupt communication to the display and can also cause wider system faults. Signs include a completely blank display or erratic behaviour alongside the F90 code.
- Power surge or electrical spike Sometimes
A sudden spike in mains voltage — from a lightning strike, a power cut restoration, or a fault elsewhere in the property — can corrupt the PCB's firmware or physically damage components, resulting in a persistent F90 communication fault after power is restored.
How to fix it
- Check your boiler model and manual DIY safe
Before doing anything else, locate your boiler's model name and serial number (usually on a label inside the front casing). Confirm the code displayed is F90 and not F9 or a different code — this makes a difference on older models and will help any engineer you call.
- Attempt a single boiler reset DIY safe
Press and hold the reset button (usually marked with a flame and cross symbol) for around 3 seconds, then release. Allow the boiler up to 2 minutes to attempt a restart. If the F90 returns immediately or within a short time, do not reset more than 2–3 times in total — repeated resets on a genuine electrical fault will not fix the underlying problem and can occasionally mask a developing issue.
- Check your mains power supply DIY safe
Ensure there has not been a recent power cut or electrical disturbance at your property. Check that the boiler's fused spur switch is turned on and that no relevant circuit breakers have tripped in your consumer unit. Restore power normally if anything has tripped, then reset the boiler once.
- Do not open the boiler casing or touch any internal wiring Gas Safe engineer
The F90 fault involves the PCB and internal wiring — these are not homeowner-serviceable components. Opening the casing and probing connectors without the correct training risks electric shock, voiding your boiler warranty, and potentially damaging components further. Leave all internal inspection to a qualified engineer.
- Call a Gas Safe registered engineer to diagnose the fault Gas Safe engineer
A Gas Safe engineer will open the casing safely, inspect the wiring harness between the PCB and the display panel for loose pins or corrosion, test the user interface panel independently, and use diagnostic tools to confirm whether the PCB itself has failed. They will then advise on whether a wiring repair, a display panel replacement, or a PCB replacement is needed. You can find a registered engineer at gassaferegister.co.uk.
Parts you may need
- Glow-worm user interface / display panel · from £95
- Glow-worm main PCB (control board) · from £280
- Wiring harness / connector loom · from £35
- Mains surge protection plug (preventative) · from £20
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£380, depending on the underlying cause.
Frequently asked questions
Is the Glow-worm F90 the same as the Vaillant F.90?
Effectively yes, for most modern Glow-worm models. Because Glow-worm has been part of the Vaillant Group since 2001, the two brands share the same control architecture on many product lines. The Vaillant F.90 is a documented PCB-to-user-interface communication fault, and it carries the same meaning on compatible Glow-worm boilers. If your boiler is an older model, it is worth double-checking your manual, as some earlier Glow-worm ranges used slightly different fault code numbering.
Can I fix a Glow-worm F90 fault myself?
No — this fault involves the boiler's PCB and internal electrical connections, which must only be worked on by a Gas Safe registered engineer. The only safe homeowner actions are to attempt a single reset and check the mains supply has not tripped. If the fault persists, you need a professional diagnosis. Attempting to open the boiler and handle the wiring yourself risks electric shock and will almost certainly void any remaining warranty.
How much does it cost to fix a Glow-worm F90 fault?
Most people with this fault pay somewhere between £120 and £380, depending on the cause. A wiring or connector repair sits at the lower end, while replacing the user interface panel typically costs £150–£300 all in. A full PCB replacement is considerably more expensive — usually £450–£700 including parts and labour — but engineers will only recommend this once they have confirmed the PCB itself is at fault rather than a cheaper component. If your boiler is over 10–12 years old and needs a new PCB, it is worth getting a new boiler quote at the same time for comparison.
Will the F90 fault clear on its own after a reset?
Occasionally a one-off power disturbance can trigger a temporary F90, and a single reset clears it permanently. However, if the code returns after resetting — or comes back within a few days — there is an underlying hardware problem with the wiring, display panel, or PCB that will not self-resolve. Repeatedly resetting the boiler in this situation is not a fix and may delay a diagnosis, leaving you without heating or hot water for longer.