Your boiler has stopped working because it thinks the hot water tank is dangerously overheating due to a faulty electrical sensor.

Burner blocked. Short circuit - cylinder temperature sensor (system boiler).

What causes this fault?

This fault occurs when the electrical sensor that monitors the temperature of your hot water cylinder fails or develops a short circuit. The boiler's control board detects an impossible reading, such as an extreme spike in heat, and shuts down the burner to prevent the water tank from dangerously overheating. The most common reason is either a faulty internal thermistor within the sensor itself or damaged wiring connecting the sensor to the boiler.