Officers pull over a great many drivers for DUI every week. Actually, the officers don't pull over drivers for DUI at first - there has to be some type of underlying traffic offense, like weaving or speeding, or it has to be a properly conducted drunk driving checkpoint. Then the officer may feel that the driver smells of alcohol, or is disheveled in appearance, has bloodshot or glassy eyes, slurred speech, or an open container of alcohol in the car. In many instances, the officer will next ask the driver to submit to field sobriety tests such as the HGN, the walk and turn, and the one leg stand.
If the driver (in the opinion of the police officer) exhibits clues consistent with DUI, the officer may deem that the driver has not passed the tests. They may administer a preliminary breath test. The preliminary breath test detects the presence of alcohol. The driver blows into a device and it registers a number with .08 being the legal limit. The problem is, these tests are sensitive to temperature, radio frequencies, and candy, mints, mouth wash or gum that the driver may have used.