What does the Main 125 fault code mean?

Your boiler is failing to detect the expected temperature rise after the flame ignites, which means it cannot safely heat your water or radiators.

How much does it cost to fix Main error code 125?

Typical repair cost for the Main 125 fault is £150-300 including labour.

Primary water temperature has not changed by 1°C within 15-30 seconds of burner lighting or within 10 minutes of burner lighting

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

Almost always a stuck diverter valve on combis.

Each noise points to a different fault.

Sludge build-up — needs flushing, not bleeding.