Your boiler has detected a faulty electrical signal from the pipe that brings water back into it, which is preventing the system from heating up correctly.

Short circuit on return sensor

What causes this fault?

This fault occurs when the internal sensor responsible for monitoring the temperature of the water coming back into the boiler fails or loses its connection. It is usually caused by the sensor's internal electrical circuit failing over time or moisture affecting the wiring, which prevents the boiler from safely calculating how much heat is needed.