What does the Remeha b25 fault code mean?

Your boiler is detecting that its internal water temperature is rising much faster than it should, so it has shut down to prevent damage.

How much does it cost to fix Remeha error code b25?

Typical repair cost for the Remeha b25 fault is £150-300 including labour.

Temperature rise too fast (dT/dt)

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.