40 High

Atag 40 Fault Code

The Atag 40 fault code means: your boiler has detected a problem with the sensor that monitors the temperature of the water returning from your radiators, preventing it from heating your home safely.. It appears on 10 Atag models in our database.

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: Return sensor error (open, short circuit, or out of range)
Typical repair cost £120-190 (including labour)

Engineer required

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

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Atag models that show the 40 fault code

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

Source

Atag 40 fault code information on this page is sourced and cross-checked against the official Atag service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026