Your boiler has detected a sensor problem that means it cannot accurately measure the temperature of the water returning to the unit, causing it to shut down for safety.

Return thermistor fault

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 provides an Impossible reading. The most common reason is that the sensor has physically degraded over time or a loose electrical connection is preventing it from communicating with the main control board.