Your boiler has detected an internal sensor failure and has stopped working to prevent damage or overheating.

Indicates a fault condition; check thermistor resistance (690ohms to 33k) on pins 3 & 4 or pins 19 & 20 to determine if there is an open or short circuit.

What causes this fault?

This fault occurs when the boiler's internal sensors, which monitor water temperature, stop sending a reliable signal to the main control board. The most common reason is that the sensor has physically degraded or developed an electrical short, meaning the boiler can no longer safely regulate its heat output. In some cases, old or damaged wiring between the sensor and the brain of the boiler is the culprit.