I use those since several years on our boats, work very well.
You can calibrate on ambient air. Pull the green plug and wave it a bit around.
It comes with an offset chart, that tells you how to correct depending on temperature and humidity. You will see when the sensor starts to go bad, you can't get a stable reading.
It usually helps to open the unit and dry it out, but that is only a temporary fix.
Time to get a new sensor.
You can calibrate on ambient air. Pull the green plug and wave it a bit around.
It comes with an offset chart, that tells you how to correct depending on temperature and humidity. You will see when the sensor starts to go bad, you can't get a stable reading.
It usually helps to open the unit and dry it out, but that is only a temporary fix.
Time to get a new sensor.