d.40 Medium

Heatline d.40 Fault Code

Also written as D40 — same fault.

The Heatline d.40 fault code means: your boiler is reporting an issue with the temperature of the water being sent to your radiators, which usually means the system is struggling to heat up correctly.. It appears on 7 Heatline models 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: Heating fl ow temperature
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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Heatline models that show the d.40 fault code

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

Source

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

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