4.2 Medium

Baxi 4.2 Fault Code

Also written as 42 — same fault.

The Baxi 4.2 fault code means: your boiler has detected an issue with its installation setup, often caused by a communication error between the boiler and its external controls or settings.. It appears on 1 Baxi model in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Installation regulations
Typical repair cost £120-250 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Baxi models that show the 4.2 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Baxi 4.2 fault code information on this page is sourced and cross-checked against the official Baxi service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026