How much does it cost to fix Vaillant error code con?

Typical repair cost for the Vaillant con fault is £150-£450 including labour.

con High

Vaillant con Fault Code

Q: What does the Vaillant con fault code mean?

Your boiler's internal computer screen and main control board are having trouble talking to each other, meaning the unit cannot function.

Also written as CON — same fault.

The Vaillant con fault code means: your boiler's internal computer screen and main control board are having trouble talking to each other, meaning the unit cannot function.. It appears on 25 Vaillant models in our database.

Technical description: no communication to mainboard | connection error display mainboard

What causes the Vaillant con fault?

This fault occurs when the communication link between the boiler's control interface and its main brain is broken, preventing any commands from being processed. The most common reason is a loose or damaged internal wiring connection, though it can also be triggered by a power surge that has damaged the delicate circuitry on one of the electronic boards. Without this digital conversation, the boiler cannot safely ignite or operate.

Typical repair cost £150-£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 con fault code

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

Source

Vaillant con 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