Your boiler has detected an issue with how the fuel is burning or how the main heat exchanger is performing, causing it to shut down for safety.

Heat exchanger monitoring fault. Check/adjust gas valve differential (page 42). Clean/replace ionisation probe & lead Part No. 5106941. Test combustion at high and low input (page 42). Check boiler control responds to flow & ret. sensors

What causes this fault?

This fault occurs when the boiler's internal sensors detect that the burner is not firing efficiently or that the main heat exchanger is struggling to transfer heat correctly. The most common reason is a worn-out ignition part that can no longer accurately read the flame, or a build-up of debris within the heat exchanger that is restricting performance. Over time, the gas-to-air mixture may also drift out of balance, leading the system to shut down as a safety precaution.