Your boiler is currently unable to read the temperature of the water returning to it, which means it has stopped working to prevent potential damage.

Central heating return NTC fault Return NTC cable defective/broken NTC faulty Check that thermistor attached correctly t

What causes this fault?

This fault occurs when the sensor responsible for monitoring the temperature of the water leaving the boiler suffers an internal electrical failure. The most common reason is that the sensor has developed a short circuit, causing it to send an impossible reading to the control board, which shuts the system down for safety. Occassionally, this can also be caused by moisture ingress or a damaged wire leading to the sensor.