Glow-worm F12 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F12 fault code mean?
The F12 code on a Glow-Worm boiler signals that the main printed circuit board (PCB) and the display PCB have lost the ability to communicate reliably with each other. When this link breaks down, the boiler's control system can no longer function correctly, so it shuts itself down as a precaution. You may notice the display flickering or going blank, the controls becoming unresponsive, and a complete loss of heating and hot water. This is a lockout fault — the boiler will not restart on its own until the underlying communication problem is identified and corrected.
General guidance only — not a substitute for professional advice. 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 damaged wiring between the PCBs Common
The wiring loom that carries signals between the main board and the display board can develop loose connectors or broken wire cores over time, particularly on boilers that have been running for a decade or more. Vibration from the pump and fan gradually works connections loose, breaking the communication link and triggering F12.
- Corroded or pulled connector plugs Common
The plug-in connectors joining the two boards are prone to corrosion, especially in boilers installed in damp locations such as kitchens or garages. A connector that has partially pulled out or whose terminals have oxidised will cause intermittent or total loss of the PCB-to-display signal.
- Faulty display PCB (user interface board) Common
On CXI, HXI and SXI models in particular, the smaller display or user-interface board is a known weak point. If the board itself has failed — often visible as burnt marks or damaged tracks — it will generate F12 alongside related codes such as F13 or F14. Replacing this board frequently resolves the fault.
- Water ingress or moisture damage Sometimes
Internal leaks from the pump, primary heat exchanger seals or pipework joints can allow moisture to reach either PCB. Even a small drip over a long period causes corrosion and short-circuits that disrupt the communication bus between the boards.
- Faulty main PCB Sometimes
Less commonly, the main board itself is the source of the problem rather than the display board or wiring. If inspection and connector re-seating do not clear the fault, and the display board tests fine, the main PCB may need to be replaced.
How to fix it
- Switch the boiler off at the programmer or room thermostat DIY safe
Turn the heating and hot water demand off before doing anything else. This reduces the risk of the fault worsening and gives the electronics a moment to power down fully.
- Attempt a boiler reset DIY safe
Locate the reset button — usually red and marked with a reset symbol on the front panel. Press and hold it for three to five seconds until the boiler tries to fire. If F12 clears and the boiler runs normally, monitor it closely over the next 24 hours. If the code returns, do not keep resetting; repeated lockouts on the same fault need professional diagnosis.
- Check for obvious signs of a water leak near the boiler DIY safe
Look underneath and around the boiler casing for drips, damp patches or watermarks. If you spot an active leak, turn the boiler off at the isolator and contact a Gas Safe engineer promptly — moisture reaching the PCBs can cause further and more expensive damage.
- Do not attempt to open the boiler or handle the PCBs yourself Gas Safe engineer
Accessing, re-seating or replacing PCBs and their wiring connectors requires the boiler casing to be removed and work carried out on live electronics and gas-carrying components. This must only be done by a Gas Safe registered engineer. Incorrect handling can cause further board damage or create a safety hazard.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
An engineer will remove the casing safely, inspect the wiring loom and connector plugs between the main PCB and the display board, check for signs of water damage or burnt components, and re-seat or replace any faulty parts. On CXI, HXI and SXI models, the display PCB is the most frequent culprit and is usually the first component replaced. If the fault persists after board replacement, the main PCB may also need attention.
Parts you may need
- Display / user-interface PCB · from £120
- Main PCB · from £180
- PCB wiring loom / connector harness · from £35
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 £150–£350, depending on the underlying cause.
Frequently asked questions
Is the Glow-Worm F12 fault dangerous?
F12 does not indicate a gas leak or combustion problem, so it does not pose an immediate safety risk. However, any boiler that has locked out should be investigated promptly. If the root cause is water damage from an internal leak, leaving it unresolved can lead to more serious electrical faults. Stop using the boiler and get an engineer in sooner rather than later.
Can I fix the F12 fault myself?
The only DIY steps worth trying are switching the boiler off and attempting a single reset. Beyond that, the repair requires opening the boiler casing and working on PCBs and wiring — tasks that should only be carried out by a Gas Safe registered engineer. Attempting it yourself risks damaging expensive components and could void any remaining warranty or boiler cover.
How much does it cost to fix a Glow-Worm F12 fault in the UK?
Most homeowners pay somewhere between £150 and £350, depending on which component needs replacing and local labour rates. A diagnostic visit plus display PCB replacement typically falls in the middle of that range. If both the display board and the main PCB need replacing, costs can push beyond £400 — at which point, on a boiler that is already 10 or more years old, it is worth discussing with your engineer whether a new boiler would be a better investment.
My Glow-Worm CXI, HXI or SXI is showing F12 and the screen has gone blank — what should I do?
A blank display is a known symptom of F12 on these models and usually points to a failed display (user-interface) PCB. The fault can appear alongside F13 or F14 codes on the same family of boilers. There is unfortunately no homeowner fix for this — you will need a Gas Safe engineer to inspect the board and its connector plugs and replace the display PCB if it has failed. These models are often 10 or more years old when this fault appears, so ask your engineer for an honest appraisal of repair versus replacement costs.