High Lockout
F42 on Glow-worm Flexicom sx

Your boiler has detected an issue with one of its internal sensors, preventing it from safely controlling your heating and hot water.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Coded resistance fault

What causes this 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 is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

F41View →

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