Your boiler has detected that its internal temperature sensors are either misaligned or not communicating correctly, preventing the system from heating safely.

Error with flow and return temperature, sensor incorrectly fitted

What causes this fault?

This fault occurs when the boiler detects that its internal temperature sensors are not responding in the way they should, often because they have come loose or were installed incorrectly. The most common reason is that a sensor has shifted away from the pipework it is meant to monitor, meaning it cannot provide an accurate reading to keep the system running safely.