Your boiler is temporarily running at a lower power setting because it has already reached its target temperature and is waiting for the system to cool down slightly before firing up again.

DIY fix possible

Check that all radiator valves are fully open to allow heat to move away from the boilerEnsure your room thermostat is set to a temperature higher than the current room temperatureMonitor the boiler to see if the code clears naturally once the heating system demands more heat