Your boiler's internal thermometer that measures the water returning from your radiators has developed a fault and can no longer read the temperature correctly.

Return temp. sensor is faulty

What causes this fault?

This fault occurs when the sensor responsible for measuring the water coming back from your radiators fails to send a reliable signal to the boiler's control board. The most common reason is that the internal electrical components of the sensor have worn out or corroded over time, or a wire has become loose or damaged. Because the boiler can no longer verify the water temperature, it shuts down as a safety precaution to prevent overheating.