Your boiler has detected an electrical fault with the sensor that monitors water returning from your radiators, causing the system to shut down for safety.

Return NTC short circuit

What causes this fault?

This fault occurs when the electrical connection within the return pipe sensor breaks down, creating a 'short circuit' that sends an incorrect signal to the boiler's computer. The most common reason is internal wear and tear of the sensor itself, though it can also be triggered by water leaking onto the electrical cables or a fault in the main control board.