Your boiler has switched its control method to the return pipe because it is detecting a potential issue with the main flow sensor.

Heating flow return regulation

What causes this fault?

This fault occurs when the boiler detects a logical error or an inconsistent reading from the flow temperature sensor. To keep your heating running safely, the boiler automatically switches to monitoring the return pipe sensor while it ignores the potentially faulty flow data. The most common reason is a failing or 'drifting' sensor that is sending incorrect electrical signals back to the main control board.