403 High

Viessmann 403 Fault Code

The Viessmann 403 fault code means: your boiler has detected an electrical issue with the internal flame sensor or its main control board, preventing it from heating your home safely.. It appears on 1 Viessmann model 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: Ionisation electrode earth fault; HBMU faulty
Typical repair cost £180-£650 (including labour)

Engineer required

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

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Viessmann models that show the 403 fault code

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

Source

Viessmann 403 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026