Guides
Ignition interlock device basics and who needs one
An ignition interlock device, usually shortened to IID, is a breath alcohol tester wired into a vehicle so the engine will not start until the driver blows a clean sample. Courts and state licensing agencies order them after many impaired driving offenses, and some drivers install one voluntarily to keep or restore driving privileges sooner. This guide explains how the device works, who is typically required to use one, what happens during a drive, what the unit records, and how an interlock differs from a personal breathalyzer you buy for yourself.
- 1Blow before startSample taken at the dash unit
- 2Engine unlocksOnly under the set point
- 3Rolling retestsAt random during the trip
- 4Log is readAt each service visit
How an ignition interlock works in a car
An interlock has two main parts. A handheld unit with a mouthpiece sits in the cabin, usually on a short cord near the dashboard, and a control module is installed under the dash and connected to the starting circuit. When the driver turns the key or presses the start button, the handheld asks for a breath sample before the starter is allowed to run.
Inside the handheld is an alcohol specific fuel cell sensor, the same family of sensor used in professional screening devices. The unit compares the alcohol in the sample with a limit set by the state program. That limit is often much lower than the 0.08 per se limit for adult drivers, and many programs set it around 0.02 or 0.025, so the device can refuse a start even when a driver would not be over the legal limit.
If the sample is below the set point, the module allows the engine to start. If it is at or above the set point, the start is blocked and the event is logged. After a failed test the unit typically enforces a short lockout period before it accepts another sample, and repeated failures can lead to longer lockouts.
Most modern units also ask the driver to follow a breath pattern, such as blowing steadily and then humming or sucking back, which helps the device confirm that a real human breath sample has been provided.
Rolling retests while you drive
An interlock does not stop checking once the engine is running. At random intervals during the trip, the device asks for another sample, called a rolling retest or running retest. The driver usually has several minutes to pull over safely or provide the sample, and programs and manufacturers instruct drivers to stop in a safe place before blowing.
A missed or failed rolling retest never shuts the engine off while the car is moving, because that would create a road hazard. Instead the event is recorded, and depending on the program the vehicle may sound the horn or flash the lights until the engine is turned off, and the driver may be required to visit the service center early.
Who is required to install an interlock
Every state has some form of ignition interlock law, but the details differ a lot. In general terms, an interlock may be ordered or required in these situations:
- After a first impaired driving conviction in states with all offender laws.
- After a repeat offense, a high BAC offense or a refusal to take a chemical test, in states that target those cases.
- As a condition of a restricted, hardship or probationary license that lets a person drive during a suspension period.
- Voluntarily, where a state lets drivers choose an interlock in exchange for a shorter period without any license.
How long the device stays installed also varies, from a few months to several years, and some programs extend the period after violations. Because these rules change and depend on the exact case, the only reliable source is your court order and your state DMV or the state agency that runs the interlock program. The NHTSA publishes background research on interlock programs, and the Governors Highway Safety Association summarizes state laws, but neither replaces your own state's rules.
This page is general information, not legal advice. If you face an interlock requirement, a licensed attorney in your state can explain how it applies to you.
What the interlock records and reports
An interlock is also a data recorder. Every attempt to start the car, every sample result, every rolling retest and every missed retest is stored with the date and time. Many units add a photo from a small camera, so the program can confirm who provided the sample, and newer devices may record GPS location.
The data is downloaded at regular service visits, often every 30 to 90 days depending on the state, and some devices report in near real time over a cellular connection. The provider sends reports to the monitoring authority, which may be the court, a probation officer or the state licensing agency. Typical items in a report include:
- Breath sample results and the time of each test.
- Failed starts and the lockouts that followed.
- Missed or failed rolling retests.
- Signs of tampering, disconnection or a dead battery.
- Missed service or calibration appointments.
Violations can extend the interlock period, lead to a license suspension or trigger a court hearing. The device also needs regular calibration at the service center to keep its sensor reading correctly, which is part of why service visits are required.
Why we never share ways around an interlock
Readers sometimes look for ways to fool an interlock. We do not publish any, and we never will. Tampering, having someone else blow, or disconnecting the device is a violation in every state program and is usually treated as a separate offense. Modern units are designed to detect these attempts through breath pattern checks, cameras and circuit monitoring, and the record reaches the monitoring authority. The interlock exists to keep a driver who has been drinking from driving, and the only reliable way to pass it is not to drink before you drive.
Interlock versus a personal breathalyzer
An interlock and a personal breathalyzer may share sensor technology, but they do different jobs. The table below sets out the main differences.
| Feature | Ignition interlock device | Personal breathalyzer |
|---|---|---|
| Who orders it | A court, state agency or voluntary program | You buy it for your own use |
| Main job | Blocks the engine above a set limit and records every event | Shows an estimated BAC for your own information |
| Installation | Wired into the vehicle by a certified provider | No installation, carried in a pocket or bag |
| Data | Logged and reported to the monitoring authority | Stays with you, sometimes in a phone app |
| Calibration | Scheduled service visits required by the program | Follows the maker's own guidance |
| Ownership | Usually leased from a provider | Owned outright |
A personal breathalyzer cannot replace an interlock that a court or state has ordered, and it cannot predict exactly what an interlock will read. Different sensors, calibration dates and timing mean the two can give different results. Some drivers who live with an interlock also keep a personal fuel cell tester to understand how alcohol from the night before is clearing, which is a reasonable use as long as the answer to any doubt is the same: do not drive.
This is an estimate. It cannot tell you whether you are safe or legal to drive. If you have been drinking, do not drive.
Where to learn more on our site
For the costs involved, read our guide on ignition interlock device cost. To understand the numbers an interlock works with, see blood alcohol concentration and the state by state overview of the legal limit. If you want a tester for your own vehicle that is not court ordered, our guide to a breathalyzer for car use explains the options, and the fuel cell breathalyzer category covers the sensor type closest to what an interlock uses.
Ignition interlock device questions
What is an ignition interlock device?
It is a breath alcohol tester connected to a vehicle's starting system that prevents the engine from starting when the driver's breath sample is at or above a limit set by the state program.
What BAC will make an interlock fail?
Each state program sets its own limit, and many set it well below 0.08, often around 0.02 or 0.025, so check your program documents for the exact figure.
What is a rolling retest?
It is a request for a new breath sample at random times while you drive, and you should pull over somewhere safe before providing it.
Will an interlock shut off my engine while I am driving?
No, a failed or missed rolling retest does not turn off a moving engine, but the event is recorded and the vehicle may sound the horn or flash the lights until you stop.
Who sees the data from my interlock?
The service provider downloads the records and reports them to the authority monitoring your case, such as the court, a probation officer or the state licensing agency.
Can I use my own breathalyzer instead of an interlock?
No, a personal breathalyzer cannot replace a court ordered or state ordered interlock, although some drivers use one alongside it for their own information.
Where can I find my state's interlock rules?
Your court order and your state DMV or the state agency that runs the interlock program are the official sources, and an attorney can explain how the rules apply to you.