Your boiler is struggling to communicate with an external control box used for managing your hot water or heating zones, preventing it from functioning correctly.

wrong configuration with accessory link box VR65 connected to combination boiler

What causes this fault?

This fault occurs when the boiler loses its digital connection to a sensor measuring the temperature of your hot water, usually located on an external control system. The most common reason is a loose electrical connection or a broken internal wire that prevents the main circuit board from receiving a temperature reading. In some cases, the sensor itself may have failed due to age or heat exposure, leaving the boiler unable to safely manage hot water production.