At random times after the engine has been started, the IID will require another breath sample. The purpose of this is to prevent a friend from breathing into the device, enabling the intoxicated person to get behind the wheel and drive away. If the breath sample isn't provided, or the sample exceeds the ignition interlock's preset blood alcohol level, the device will log the event, warn the driver and then start up an alarm (e.g., lights flashing, horn honking, etc.) until the ignition is turned off. A common misconception is that interlock devices will simply turn off the engine if alcohol is detected; this would, however, create an unsafe driving situation and expose interlock manufacturers to considerable liability. Modern ignition interlock devices use an ethanol-specific fuel cell for a sensor. A fuel cell sensor is an electrochemical device in which alcohol undergoes a chemical oxidation reaction at a catalytic electrode surface (platinum) to generate an electric current. This current is then measured and converted to an alcohol equivalent reading. Although fuel cell technology is not as accurate or reliable as infrared spectroscopy technology used in evidentiary breathalyzers, they are cheaper and tend to be more specific for alcohol.
The devices keep a record of the activity on the device and the interlocked vehicle's electrical system. This record, or log, is printed out or downloaded each time the device's sensors are calibrated, commonly at 30, 60, or 90-day intervals. Authorities may require periodic review of the log. If violations are detected, then additional sanctions can be implemented.
Periodic calibration is performed using either a pressurized alcohol–gas mixture at a known alcohol concentration, or with an alcohol wet bath arrangement that contains a known alcohol solution. The costs of installation, maintenance and calibration are generally paid by the offender, and typically are about $75 per month.
Among manufacturers of IIDs are Smart Start Inc., LifeSafer Interlock, SOS, Ignition Interlock Systems, Intoxalock and Monitech. A list of federally-approved IID devices is maintained by the National Highway Traffic Safety Administration in their NHTSA Conforming Products List.
Many countries are requiring the ignition interlock as a penalty for drivers convicted of driving under the influence, especially repeat offenders. Most U.S. states now permit judges to order the installation of an IID as a condition of probation; for repeat offenders, and for first offenders in some states, installation may be mandated by law. (MADD) launched a highly publicized campaign advocating mandatory IID installation for all first offenders . Some politicians in Sweden, Japan, Canada, the U.S. and other countries have called for such devices to be installed as standard equipment in all motor vehicles sold.