BoilerFaultCodesGlow-worm S.42 Fault Code
Also written as S42 — same fault.
The Glow-worm S.42 fault code means: your boiler has temporarily stopped providing heating or hot water because an external control or smart accessory is preventing it from firing up.. It appears on 27 Glow-worm models in our database.
Technical description: Accesory Module is blocking burner operation
What causes the Glow-worm S.42 fault?
This fault occurs when an external device connected to the boiler, such as a smart thermostat or a wireless receiver, sends a signal to stop the burner from firing. The most common reason is a loss of communication between your thermostat and the boiler, often caused by dead batteries or a signal interference. Essentially, the boiler is waiting for a 'go' signal from your controls that never arrives or is being actively blocked.
DIY fix possible
Check your external thermostat or smart home app to ensure the heating is actually turned on.Replace the batteries in your room thermostat or programmable controller.Restart your internet router if you use a smart heating system like Hive, Nest, or MiGo.
Find a Gas Safe engineer
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A cover plan would handle faults like S.42: no call-out fees, no surprise bills.
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Glow-worm models that show the S.42 fault code
Click a model for the full fault code list and model-specific repair notes.
- Betacom4 Repair: £120-250
- Compact Repair: £150-300
- Easicom 2 24c Repair: £150-300
- EASICOM 3 25r Repair: £150-300
- Easicom3 Combi Repair: £150-250
- Easicom3 System Repair: £150-300
- Energy Repair: £150-300
- Energy Regular Repair: £150-300
- Energy System Repair: £150-300
- Energy7 Combi Repair: £150-300
- Energy7 Regular Repair: £120-250
- Energy7 System Repair: £150-300
- Essential Repair: £150-300
- Flexicom 18sx Repair: £100-250
- Flexicom 24c / 28c Repair: £150-300
- Flexicom 30sx Repair: £100-250
- Flexicom cx Repair: £60-180
- MicraCom Repair: £120-250
- Sustain 12s Repair: £120-250
- Sustain 15s Repair: £120-250
- Sustain 18s Repair: £120-250
- Ultimate 3 25R Repair: £150-300
- Ultimate3 Combi Repair: £150-300
- Ultracom 2 CXI Repair: £60-180
- Ultracom 24cx 30cx Repair: £60-180
- Ultracom 30cxi Repair: £60-180
- Ultracom 38cxi Repair: £60-180
Source
Glow-worm S.42 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 31 August 2026