Your boiler has sensed that the water returning to it is too hot, which suggests that the heat is not being effectively pumped away through your radiators.

Return temperature too high flow sensor temperature).

What causes this fault?

This fault occurs when the water coming back into the boiler is nearly as hot as the water going out, which prevents the heat exchanger from cooling down. The most common reason is a blockage in the system or a failing pump that isn't pushing the water through your radiators quickly enough. It can also be caused by a build-up of sludge or debris trapped in the boiler's internal filter.