Glow-worm F11 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F11 fault code mean?
The F11 fault code on a Glow-Worm boiler indicates a communication breakdown between the User Interface PCB (the board behind the controls and display) and the Main PCB (the board that governs the boiler's core operation). When these two boards lose the ability to talk to each other reliably, the boiler locks out to prevent unsafe operation. On CXI, HXI and SXI models this can also cause the display to go completely blank. If you have a Flexicom model, F11 can additionally point to a fault with the flow or return thermistors — the temperature sensors that monitor water moving through the heat exchanger — rather than a pure PCB-to-PCB communication issue.
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
- Trapped air in the heating system Common
An airlock is the most frequently reported trigger for the F11 fault. Air pockets disrupt water circulation, cause pressure fluctuations, and can produce erratic sensor readings that confuse the main PCB into flagging a communication fault. This is particularly common after any work has been done on the system, such as draining down radiators or replacing a component.
- Loose or corroded wiring connections between the two PCBs Common
The ribbon cables and plug-in connectors linking the User Interface PCB to the Main PCB can work loose over time, especially on boilers approaching ten years old where repeated heating and cooling cycles cause gradual movement. Oxidised or damaged connector pins produce the same intermittent signal loss as a faulty board itself, and are often found to be the root cause once an engineer inspects the wiring loom.
- Faulty or failed Main PCB Sometimes
If the wiring is confirmed to be intact and secure, the Main PCB itself may have developed an internal fault — failed capacitors, cracked solder joints, or component damage from a voltage spike being the most typical causes. A defective Main PCB on these models can also generate the related F12, F13 and F14 fault codes. Engineers will confirm this with continuity and voltage tests before recommending a replacement board.
- Faulty flow or return thermistor (Flexicom models) Sometimes
On Flexicom boilers specifically, F11 can be triggered by a thermistor that is reading outside its expected range or has failed altogether. The main PCB interprets an out-of-range temperature signal as a system fault and locks the boiler out. An engineer will check resistance values across the thermistors to confirm whether either sensor needs replacing.
- Faulty User Interface PCB Rare
Less commonly, the fault originates in the User Interface board rather than the Main PCB. Physical damage, moisture ingress, or component failure on the display board can interrupt the communication signal. This is typically ruled out after the main board and wiring have been checked.
How to fix it
- Check your radiators for airlocks and bleed them if needed DIY safe
Turn off the boiler and let the system cool down. Use a radiator bleed key to open the bleed valve on each radiator, starting with the ones furthest from the boiler. Release any trapped air until water flows steadily, then close the valve. Check the system pressure on the boiler's pressure gauge afterwards — it should sit between 1 and 1.5 bar. If it has dropped below 1 bar, top it up via the filling loop.
- Check and restore system pressure if low DIY safe
Locate the filling loop — usually a braided flexible hose with one or two valves beneath the boiler. Slowly open the valve(s) while watching the pressure gauge until it reads approximately 1 to 1.2 bar. Close the filling loop valves fully before starting the boiler again. Do not pressurise above 1.5 bar.
- Attempt a single boiler reset DIY safe
Once you have addressed any airlock or low-pressure issue, press the reset button (or follow the reset procedure in your boiler's user guide) once. Wait to see whether the boiler fires up and runs normally. If the F11 code returns immediately or the boiler fails to light, do not keep resetting — further attempts without identifying the underlying cause are unlikely to help and could mask important fault information for the engineer.
- Turn off the boiler and call a Gas Safe registered engineer Gas Safe engineer
If the F11 persists after the steps above, the fault lies in the boiler's electrical system and requires professional diagnosis. Do not attempt to access or inspect the PCBs, wiring connectors, or any internal components yourself. A Gas Safe engineer will use a multimeter to test voltages and continuity across the wiring loom and both PCBs, identify whether the fault is in the connections, the Main PCB, the User Interface PCB, or — on Flexicom models — a thermistor, and carry out the appropriate repair safely.
Parts you may need
- Glow-Worm Main PCB (model-specific) · from £220
- Glow-Worm User Interface PCB · from £95
- Flow or return thermistor (NTC sensor) · from £18
- 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 £250–£500, depending on the underlying cause.
Frequently asked questions
Can I keep resetting my Glow-Worm boiler when F11 appears?
It is best to try a single reset after addressing any obvious issues such as low pressure or trapped air. If the fault code comes straight back, repeated resets will not resolve an electrical communication fault and may complicate the engineer's diagnosis. Switch the boiler off and book a Gas Safe engineer rather than pressing reset multiple times.
How much does it cost to fix a Glow-Worm F11 fault in the UK?
Most F11 repairs fall in the range of £250 to £500 including labour and VAT. If the cause turns out to be a loose wiring connector or a straightforward thermistor swap, costs sit towards the lower end. A full Main PCB replacement — parts plus labour — typically costs £350 to £500, though prices can exceed this on older or less common models where the board is harder to source. If your boiler is over ten years old and the repair quote approaches or exceeds half the cost of a new boiler, it is worth getting a replacement quote at the same time.
Why does my Glow-Worm display go blank when F11 is shown?
On CXI, HXI and SXI models in particular, a loss of communication between the User Interface PCB and the Main PCB can cause the display to go completely dark rather than showing the code in the usual way. If your screen is blank and the boiler is not firing, F11 is one of the likely culprits. A Gas Safe engineer can confirm this using diagnostic tools even without a visible fault code on the display.
Is the F11 fault different on a Glow-Worm Flexicom?
Yes, it can be. While F11 on most Glow-Worm models points squarely to a PCB communication problem, on Flexicom boilers the code can also be triggered by a faulty flow or return thermistor. These small temperature sensors are significantly cheaper to replace than a PCB, so if you have a Flexicom it is worth making sure the engineer checks the thermistors before assuming the main board needs replacing.