Your boiler is currently running in a special engineer-only testing mode for checking gas emissions and is not experiencing a mechanical fault.

combustion analysis mode (CO mode)

What causes this fault?

This code appears when the boiler has been manually put into a dedicated testing mode, typically by an engineer turning a selector switch or pressing a specific button sequence. It is designed to override normal thermostat controls so the boiler runs at a fixed rate, allowing for accurate measurement of gas combustion levels. It is not triggered by a component failure, but rather by the system being told to perform a functional test.