Your boiler has detected an issue with the sensor that monitors the water temperature in your main heating circuit, which may prevent your radiators from warming up correctly.

Main zone water thermostat

What causes this fault?

This fault occurs when the boiler's internal computer receives an incorrect or missing signal from the sensor responsible for monitoring the heat of the water sent to your radiators. The most common reason is that the sensor has developed a technical fault over time or the electrical connection between the sensor and the main control board has become loose or damaged.