Your boiler has detected a fault with the sensor that monitors water temperature returning from your radiators, preventing the system from heating up properly.

The heating return thermistor is short circuited.

What causes this fault?

This fault occurs when the sensor responsible for measuring the temperature of the water coming back from your radiators fails or develops an electrical fault. The most common reason is that the internal components of the sensor have degraded over time or moisture has compromised the electrical connection, causing it to send a faulty signal to the main control board.