T.03 High

Vaillant T.03 Fault Code

Also written as T03 — same fault.

The Vaillant T.03 fault code means: your boiler's fan is struggling to start or run at the correct speed, which is preventing the boiler from firing up for safety.. 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: Fan

What causes the Vaillant T.03 fault?

This fault occurs when the boiler's control system detects that the fan is not spinning at the required speed to safely clear exhaust gases. The most common reasons are a mechanical failure within the fan motor itself or a communication issue where the main circuit board is unable to send power or instructions to the fan.

Typical repair cost £250-£450 (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 T.03 fault code

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

Source

Vaillant T.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 27 April 2026