Your boiler has detected a fault with its temperature sensor, which means it cannot tell how hot the water is getting and has shut down as a safety precaution.

Appears if a problem exists with the primary thermistor.

What causes this fault?

This fault occurs when the boiler's internal thermometer, known as a thermistor, stops sending a reliable reading to the main control board. The most common reason is that the sensor has physically failed due to age or internal wear, though loose or damaged wiring between the sensor and the circuit board can also break the signal.