"Overheating"??. Not very helpful, is it. Although I would guess it's the water temperature rather than anything in the heat exchanger.
Most likely a simple thermistor sensor (sort of thermostat).
There'll be one clipped onto one of the water pipes in the boiler.
If it's faulty, it will be telling the boiler that the temperature is too high... when it actually isn't.
Simple thing to buy and replace, IF you know what to look for.
It may not be that, so it really does need a good look-at from a boiler guy.
If I'm right, then there is no way for you to clear the error yourself until the sensor has been replaced.
https://www.digikey.co.uk/en/product-highlight/a/amphenol-advanced-sensors/ntc-thermistor-clip-on-pipe-sensor?utm_adgroup=General&utm_source=google&utm_medium=cpc&utm_campaign=Dynamic%20Search&utm_term=&productid=&gclid=EAIaIQobChMIhf7j6KOk6QIViK3tCh1PNwl3EAAYASAAEgIkg_D_BwE
It's only a guess though. I'm afraid I'm no boiler tech ;o(