What does the Vaillant S.3 fault code mean?

Your boiler is struggling to light the burner to provide heating, which means your radiators will stay cold.

How much does it cost to fix Vaillant error code S.3?

Typical repair cost for the Vaillant S.3 fault is £100-280 including labour.

Check if other gas appliances, like a hob, are working to confirm gas supply

Heating mode error – ignition problem

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Lockout fault with a reset-needed pattern.

Older boilers — thermocouple, draught, gas supply.

Sludge build-up — needs flushing, not bleeding.