Your boiler has detected that the temperature sensor used to monitor the water inside the unit is broken or sending incorrect signals.

Boiler sensor fault detected by diagnostic feature.

What causes this fault?

This fault occurs when the internal electronic sensors can no longer accurately measure the temperature of the water moving through the boiler. The most common reason is that the sensor's internal resistance has failed due to age or heat stress, but it can also be triggered by loose or corroded electrical connections between the sensor and the main control board.