Your boiler is operating normally and performing a standard adjustment to its heating power level.

The burner operates with an adjustable heat input in heating or domestic hot water mode

What causes this fault?

This fault occurs when the boiler's internal diagnostic program fails to complete its high-performance test correctly. It is usually triggered manually by an engineer during a service or installation, and the error suggests that the system couldn't maintain the required heat output to finish the check. The most common reason is a sensor reading an unexpected temperature or the test being interrupted before the sequence was finished.