Your boiler has detected that the water temperature readings are stuck or not changing as they should, so it has stopped running to prevent potential damage.

Flow/return temperature sensor fixed value

What causes this fault?

This fault occurs when the boiler's control board detects that the temperature sensors are giving constant or illogical readings that don't change as the water heats up. The most common reason is that the flow sensor has either physically failed internally or has lost its electrical connection to the main circuit board. Without accurate temperature data, the boiler shuts down as a safety precaution to prevent overheating.