AlcoSense Zenith Unit, the LCD screen fogs up a little bit.

If the screen fogs up.
You can either just buy a new one (Non-pressure sensor) and change it yourself or send it back for calibration if it's a pressure sensor. 
 
Reason being, some of the units do fog up, there's actually not much we can do about it. 
This is due to the gap from where the breath is being sampled to the sensor but because the LED screen is quite close to the sensor, sometimes it can fog up especially if being used in a cold environment.