Your boiler is continuing to run its internal pump after the heating has switched off to safely cool down the heat exchanger.

DIY fix possible

Check that at least one radiator (usually one without a plastic thermostatic valve) is turned fully open to allow water flow.Ensure your external heating controls or room thermostat are set to your desired temperature.Wait for the boiler to complete this cycle; it is a normal safety function and the code will disappear once the system has cooled.