Your boiler has detected a critical internal error that prevents it from starting safely, so it has shut down to protect itself.

Boiler fault. Call engineer.

What causes this fault?

This fault occurs when the boiler's main temperature sensor, which monitors the heat of the water inside the unit, stops sending a reliable electronic signal to the control board. The most common cause is the sensor's internal components wearing out over time, though it can also be triggered by a loose or damaged connection in the internal wiring. Because the boiler no longer knows how hot the water is, it shuts down as a safety precaution to prevent internal damage.