Yea, the E03 means a calibration failure due to not mounting it in your case.
The reported temp is mostly too high for radiator thermostats since it is so close to the radiator itself. I use mine with an automation ( 
Advanced Heating Control - Blueprints Exchange - Home Assistant Community (home-assistant.io)) and an external temperature sensor/thermometer somewhere in the room. That way I get consistent temperatures all around the house.
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