An ignition interlock device (IID) is a breath-alcohol sensor wired into a vehicle's ignition. The driver must blow a sample below the threshold (typically 0.025% BAC) before the engine will start, and the device requires periodic rolling re-tests while driving.
Missouri requires an IID for any restricted or hardship driving privilege after a DWI-related suspension. The Limited Driving Privilege (LDP) and the Restricted Driving Privilege both have IID conditions. Refusal cases and repeat offenders face longer IID requirements.
Illinois requires an IID through the Monitoring Device Driving Permit (MDDP) for first-time DUI defendants during the Statutory Summary Suspension. Repeat offenders need an IID through the Restricted Driving Permit issued by the Secretary of State.
IID violations — failed tests, missed rolling re-tests, tampering — get reported automatically to the state and can extend the underlying suspension or trigger new criminal charges.