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Drivers convicted of drunk driving are required in many communities to purchase and maintain an ignition interlock for a set period of time (usually years) after their conviction. This is basically a breathalyzer that connects to the ignition, and must return a negative test result before the car can be started.

These devices have to be expensively re-calibrated at intervals (sometimes monthly), and can not only tell if you're over the legal limit, but some of them can even tell if your'e you. So if you have a friend breathe into it, or just squeeze a bottle of air into the collector, it won't work!

Some ignition interlocks have advanced features, such as only allowing the car to start during certain intervals. (It is common for drunk drivers' privileges to be restricted to going to work and treatment only.)

Note: This is totally different from a transmission interlock.