A vehicle ignition interlock system includes a breath analyzer and a controller operably connected to the breath analyzer and to an ignition system of the vehicle. The breath analyzer detects the breath alcohol level of a vehicle operator and is configured to prevent vehicle ignition if a breath alcohol level is greater than or equal to a threshold value. The controller requires the vehicle operator to periodically take breath analyzer "retests" after vehicle ignition in order to allow vehicle operation to continue. The breath analyzer also includes a mouth contamination sensor that is configured to distinguish between contaminants in the mouth of a vehicle operator and alcohol contained within a deep lung breath sample of the vehicle operator. The controller may require a breath alcohol retest of the vehicle operator via the breath analyzer in response to detecting the presence of a contaminant within the mouth of the vehicle operator.