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 12 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 31 August 2026