How much does it cost to fix Viessmann error code A3?

Typical repair cost for the Viessmann A3 fault is £120-250 including labour.

A3 High

Viessmann A3 Fault Code

Q: What does the Viessmann A3 fault code mean?

Your boiler has switched itself off because a sensor inside detects that the exhaust system or internal temperature isn't configured exactly as it should be.

The Viessmann A3 fault code means: your boiler has switched itself off because a sensor inside detects that the exhaust system or internal temperature isn't configured exactly as it should be.. It appears on 1 Viessmann model in our database.

Technical description: Burner blocked, flue gas temperature sensor not positioned correctly

Typical repair cost £120-250 (including labour)

Engineer required

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

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →

Viessmann models that show the A3 fault code

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

Vitodens 100-W Repair: £120-250

Source

Viessmann A3 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 27 April 2026