While these units are incredibly handy and helpful, the downside is that they aren’t going to work with every make and model of car. Many companies offer up a list of cars that they have found that their devices do not work with, so be sure to consult these lists before making your purchase. Designs that use the OBD II port are going to work on models of American cars that were manufactured after 2003, or European or Japanese cars that were made after 2008 or so. If you are driving a pre-2003 car, there are still going to be some options that you can select from. Some designs don’t use the diagnostic plug at all, and simply plug into the cigarette outlet in your car. However, this means that you won’t be able to have a readout on the health of your car as you drive, but you will still be able to read things like speed, direction, and time.