Your boiler has sensed that there isn't enough water moving through the system to heat up safely, so it has shut down to protect itself.

waiting time: no water in system, temperature (gradient error) rise flow / return sensor too high

What causes this fault?

This fault occurs when the boiler's internal sensors detect that the water temperature is rising far too quickly, which usually means the water is trapped and not flowing. The most common reasons are a failed circulation pump that can't move the water or a blockage caused by a build-up of sludge or air within the pipework. It acts as a safety measure to prevent the heat exchanger from overheating and sustaining permanent damage.