Your boiler has detected an issue with a temperature sensor that monitors the water returning from your radiators, often resulting in the system shutting down for safety.

Return Thermistor Fault

What causes this fault?

This fault occurs when the boiler's control board loses communication with the sensor that monitors the temperature of the water coming back from your radiators. The most common reason is that the sensor itself has developed an internal electrical failure or the wiring connecting it to the main computer has become loose or damaged. Because the boiler can no longer safely track how much heat is being removed from the system, it shuts down to prevent overheating.