Your boiler is having trouble communicating with its internal temperature sensors, which means it cannot accurately measure how hot the water is and has stopped working as a safety precaution.

Flow and return sensor connection fault. Fault may occur after several minutes of boiler operation.

What causes this fault?

This fault occurs when the boiler's control board receives conflicting or missing data from the sensors that measure water temperature entering and leaving the unit. The most common reason is that one of these sensors has physically failed or the electrical wiring connecting them has become loose or damaged over time. Because the boiler cannot accurately track how hot the water is, it shuts down to prevent the system from overheating.