Your boiler has sensed that the water returning from your radiators is too hot, which usually means the heat isn't being transferred out of the system properly.

Return temperature too high flow sensor temperature).

What causes this fault?

This fault occurs when the water traveling back into the boiler is nearly as hot as the water leaving it, usually because the heat isn't being released into your rooms. The most common reasons are a pump failure that isn't moving the water fast enough, or a blockage in the system like trapped air or sludgy radiators that prevents proper circulation.