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.
The unit can either be returned to us for calibration.
The sensor may just be loose and needs to be re-inserted.
Remove the sensor cover, take out the sensor chip and then put it back in. This may fix the error.