Possible Cause 1:

The unit needs to be re-calibrated. If you have had the breathalyser for more than 6 months and / or have used it 200-500 times already, it is probably due for a calibration. 

Solution 1:

The unit can either be returned to us for calibration.

For Alcosense Zenith models only:
Possible Cause 1 :

The sensor may just be loose and needs to be re-inserted. 

Solution 1:

Remove the sensor cover, take out the sensor chip and then put it back in. This may fix the error.