BoilerFaultCodesGlow-worm F42 Fault Code
The Glow-worm F42 fault code means: your boiler has detected an issue with one of its internal sensors, preventing it from safely controlling your heating and hot water.. It appears on 3 Glow-worm models in our database.
Technical description: Coded resistance fault
What causes the Glow-worm F42 fault?
This fault occurs when the boiler's main control board cannot identify the specific model it is installed in due to an incorrect or corrupted electrical resistance reading. The most common reason is a failure of the small coding resistor plugged into the board, or a communication error within the wiring harness. Without this data, the boiler shuts down as a safety precaution because it doesn't know its own operating parameters.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Glow-worm models that show the F42 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F42 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