Your boiler has stopped working because it thinks the water returning to the unit is much hotter than it actually is, causing it to shut down for safety.

Return NTC short circuit. NTC defective, short circuit in cable harness, cable/casing short cut

What causes this fault?

This fault occurs when the sensor responsible for monitoring the temperature of water coming back into the boiler develops an electrical short circuit. The most common reason is that the sensor itself has suffered an internal failure or natural wear and tear, leading it to send an incorrect signal to the main control board. In some cases, water damage or a loose connection within the internal wiring can also cause this electrical error.