There are a few reasons that the breathalyser is not switching on

  1.  Weak/Flat batteries.
    Try replacing with a new set of Alkaline or Lithium batteries. They must be non-heavy duty batteries. Read more on What batteries can I use with my breathalyser?

    For Alcosense Pro breathalyser models:
    Kindly flip the cover of the battery over to see the correct direction of terminals before placing the batteries. 
    While it seems that both the negative parts got in first of the new unit due to the spring located inside, this is not the case.

  2. Calibration
    The unit may be due for calibration - especially if it hasn't been calibrated in the last 6 months. You can book your unit in for a calibration here.

  3. Chipset problem
    If you have replaced the batteries and/or if the unit is brand new or calibrated in the last 6 months then there could be an issue with the chipset. Please contact us by clicking on "New Support Ticket" above for further assistance.