Your boiler has detected a faulty temperature sensor which means it can no longer accurately monitor how hot the water is getting.

Lights: On Flash On. Unplug thermistor, check if thermistor resistance is between 0.5kΩ & 20kΩ. Plug in thermistor, leave 8-way connector unplugged, check resistance at D between 0.5kΩ & 20kΩ.

What causes this fault?

This fault occurs when the internal temperature sensor fails to send a reliable signal to the main control board, often due to internal corrosion or electrical fatigue. Because the boiler can no longer track how hot the water is, it shuts down as a safety precaution to prevent the system from overheating. In some cases, the problem can also be caused by loose or damaged wiring that has broken the connection between the sensor and the brain of the boiler.