BoilerFaultCodesGlow-worm F4 Fault Code
The Glow-worm F4 fault code means: your boiler has attempted to start several times but is unable to ignite its internal flame, so it has shut down as a safety precaution.. It appears on 20 Glow-worm models in our database.
Technical description: Ignition fault, boiler fialed to light
What causes the Glow-worm F4 fault?
This fault occurs when the boiler successfully lights but then loses its flame while it is supposed to be running. It is often caused by a dirty or worn-out sensor that can no longer 'see' the flame, or by an unstable gas supply that prevents the fire from staying lit. In some cases, internal debris or a failing control board can also interrupt the combustion process.
This fault can be more frequent during extreme cold spells if the external gas regulator is affected by the temperature or if a high demand for heating puts extra strain on older components.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
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A cover plan would handle faults like F4: no call-out fees, no surprise bills.
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Glow-worm models that show the F4 fault code
Click a model for the full fault code list and model-specific repair notes.
- 18si Repair: £120-280
- 38cxi Repair: £150-350
- Ci Plus Repair: £120-280
- Ci Range Repair: £120-280
- CXi Repair: £150-300
- Flexicom 18sx Repair: £150-300
- Flexicom cx Repair: £150-350
- Flexicom CXi Repair: £150-350
- Flexicom hx Repair: £150-300
- Flexicom hxi Repair: £120-220
- Flexicom sx Repair: £150-350
- Jaguar Combi Repair: £120-200
- Ultracom 2 CXI Repair: £150-350
- Ultracom 2 HXI Repair: £150-350
- Ultracom 24cx 30cx Repair: £150-300
- Ultracom 35 Store Repair: £150-350
- Ultracom cxi Repair: £150-300
- Ultracom hxi Repair: £150-300
- Ultracom sxi Repair: £150-300
- Xtramax HE Repair: £150-350
Source
Glow-worm F4 fault code information on this page is sourced and cross-checked against the official Glow-worm service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026