Your boiler has detected a problem with the sensor that monitors the temperature of the water returning to the unit, which is preventing it from running safely.

Return thermistor fault

What causes this fault?

This fault occurs when the boiler's internal computer receives an incorrect or missing signal from the sensor that monitors water temperature coming back from your radiators. The most common reason is that the sensor itself has worn out or developed a spiritual electrical fault, though it can also be triggered by loose or damaged wiring between the sensor and the main control board.