Your boiler has detected an internal fault that is preventing it from operating, and it requires a professional service technician to diagnose and repair it safely.

Boiler fault. Call engineer.

What causes this fault?

This fault occurs when the boiler's internal computer receives an incorrect or missing signal from the sensor that monitors hot water leaving the unit. The most common reason is that the sensor has physically failed or developed a short circuit, though it can also be caused by loose or damaged electrical connections between the sensor and the main control board.