Flashing 40 Low

Heatline Flashing 40 Fault Code

Also written as FLASHING40 — same fault.

The Heatline Flashing 40 fault code means: your boiler is struggling to work out the outside temperature because there is a problem with the external sensor or its wiring.. It appears on 2 Heatline models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Fault on Outdoor sensor
Typical repair cost £70-140 (including labour)

Engineer required

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

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →

Heatline models that show the Flashing 40 fault code

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

Source

Heatline Flashing 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 27 April 2026