Your boiler's internal temperature sensor has stopped sending a reliable signal, which means the system cannot safely monitor how hot the water is getting.

Lights: On * On Off. Possible causes: Thermistor resistance out of range (0.5kΩ - 20kΩ) or wiring fault between PCB and thermistor.

What causes this fault?

This fault occurs when the boiler's internal thermometer, called a thermistor, fails to measure the water temperature correctly. The most common reason is that the sensor has physically degraded over time or developed an electrical short, though it can also be caused by loose or corroded wiring to the main control board.