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 heating because an external add-on or control accessory is preventing it from firing up.. It appears on 12 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 control device, such as an additional wiring center or a specific zone controller, tells the boiler to stop running for safety or logic reasons. The most common reason is a communication error between the main boiler brain and an add-on expansion module that manages extra heating zones. It suggests that while the boiler itself may be healthy, the external parts controlling it have detected a conflict or a hardware failure.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Glow-worm models that show the S.42 fault code
Click a model for the full fault code list and model-specific repair notes.
- Flexicom 18sx Repair: £100-250
- Flexicom cx Repair: £60-180
- Flexicom hx Repair: £100-250
- Flexicom sx Repair: £100-250
- Ultracom 2 CXI Repair: £60-180
- Ultracom 2 HXI Repair: £60-180
- Ultracom 2 SXI Repair: £100-250
- Ultracom 24cx 30cx Repair: £60-180
- Ultracom 35 Store Repair: £60-180
- Ultracom cxi Repair: £60-180
- Ultracom hxi Repair: £60-180
- Ultracom sxi Repair: £100-250
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 27 April 2026