Your boiler is running a specialized internal test to ensure its safety sensor correctly shuts the system down if it gets too hot.

Test program for the safety temperature limit check; appliance heats up by by-passing a regulating shut down through the flow regulator until the shut off temperature of 97 °C is achieved.

What causes this fault?

This is not a mechanical failure, but rather a deliberate test mode triggered by a heating engineer during a service. It occurs when the boiler is commanded to bypass its normal temperature controls to verify that the safety sensors will correctly shut the system down if it reaches a high heat threshold. Once the boiler reaches the target temperature and confirms the safety limit works, the test concludes and the boiler returns to normal operation.