Your boiler's internal computer has detected a faulty or damaged temperature sensor, preventing the system from accurately measuring how hot the water is.

Controls without flow temperature sensor (low loss header) - System flow temperature sensor shorted out

What causes this fault?

This fault occurs when the electrical signal from the temperature sensor is interrupted or compromised, usually due to an internal short circuit. The most common reason is that the sensor's internal components have failed over time or moisture has seeped into the wiring connections. Because the boiler can no longer safely monitor how hot the water is, it shuts down to prevent overheating.