What does the Viessmann 25 fault code mean?

Your boiler is overheating and has shut itself down to prevent damage because the internal water temperature has reached a dangerously high level.

How much does it cost to fix Viessmann error code 25?

Typical repair cost for the Viessmann 25 fault is £180-450 including labour.

Boiler operates with high boiler water temperature

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

Each noise points to a different fault.

Sludge build-up — needs flushing, not bleeding.

Almost always a stuck diverter valve on combis.