F40 High

Vaillant F40 Fault Code

The Vaillant F40 fault code means: your boiler has temporarily lost its electrical power supply or experienced a fluctuation in the mains voltage.. It appears on 2 Vaillant 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: Electrical supply fault
Typical repair cost £90-180 (including labour)

DIY fix possible

Check that the fused spur switch next to the boiler is turned on.Check your household consumer unit (fuse box) to see if a trip switch has activated.Try resetting the boiler using its reset button once power is verified.

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Vaillant models that show the F40 fault code

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

Source

Vaillant F40 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.

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