F.1 High

Vaillant F.1 Fault Code

Also written as F1 — same fault.

The Vaillant F.1 fault code means: your boiler has stopped working because it cannot correctly measure the temperature of the water returning to the unit due to a faulty sensor or connection.. It appears on 27 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: Return–NTC: NTC broken, NTC cable broken, Defective connection at NTC, Defective connection at electronics | Cause: Return–NTC cable defective/broken NTC faulty

What causes the Vaillant F.1 fault?

This fault occurs when the boiler's internal computer loses communication with the sensor that monitors the temperature of the water coming back from your radiators. The most common reason is that the sensor itself has failed due to age or electrical failure, though it can also be caused by a loose connection or damaged wiring. Because the boiler can no longer safely monitor heat levels, it shuts down as a protective measure.

Typical repair cost £90-160 (including labour)

Engineer required

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

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Vaillant models that show the F.1 fault code

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

Source

Vaillant F.1 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 27 April 2026