F.03 High

Vaillant F.03 Fault Code

Also written as F03 — same fault.

The Vaillant F.03 fault code means: your boiler has lost communication with the temperature sensor in its internal hot water storage tank, meaning it cannot safely monitor or heat your hot water.. It appears on 4 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: Tank-NTC: NTC broken; NTC cable broken; Defective connection at NTC; Defective connection at electronics | Tank- NTC cable defective/ broken NTC faulty
Typical repair cost £90-180 (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.03 fault code

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

Source

Vaillant F.03 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 7 July 2026