A number of alcohol breath detectors used by the National Police Agency in Japan have been giving false readings. They even gave falsely high readings for people who had consumed absolutely no alcohol.
Drivers have been arrested for drunk driving based on false or incorrect readings from the machines; no one knows how many innocent victims have been falsely convicted. About 200 of the detectors have been used by traffic police. Officials believe that readings may have been effected by their use in cold weather, but are unsure.
Alcohol breath detectors don’t actually measure blood alcohol content (BAC), but indirectly estimate it. The only test of BAC is through drawing a sample of blood and performing laboratory tests on it. However, that is rarely done.