What Is an Ignition Interlock Device and How Does It Work?

An ignition interlock device is a breathalyzer wired into your car’s ignition that prevents the engine from starting if it detects alcohol on your breath. Set points are usually far below the legal driving limit, typically between 0.02 and 0.025 blood alcohol concentration compared to the 0.08 standard for a DUI arrest. As of March 2026, 39 states require one after a first DUI conviction.¹

How the Device Works

The unit splices into the wiring between your battery and ignition system, creating a circuit the device controls. Inside the handset, a fuel cell sensor reacts chemically when ethanol molecules hit it. If the breath sample you provide meets or exceeds the programmed threshold, the device blocks the ignition signal and the engine stays off. Under federal model specifications published by the National Highway Traffic Safety Administration, devices are tested at a 0.02 g/dL set point and must be ready to use within three minutes of being powered on.²

Passing the first test only gets you moving. Once you’re on the road, the device demands random rolling retests to confirm you haven’t started drinking after pulling away. NHTSA specifications call for the first retest within five to seven minutes of engine start, and most states give you about six minutes to provide a valid sample once the device signals.² A failed rolling retest will not shut off your engine in traffic, which would be a safety hazard. Instead, the device triggers warnings like honking or flashing lights and logs the failure for your monitoring agency.

Most units require a specific blowing technique: a sustained breath at a particular pressure, and on some models you must hum at the same time. The pattern is hard to replicate with an air pump or balloon, and getting the technique wrong produces an invalid sample, which the device treats the same as a refusal.

Why Workarounds Don’t Work

Interlocks are built to catch cheating. The internal data logger records every start attempt, every breath result, and signs of tampering such as hot-wiring or push-starting a manual transmission vehicle. Many newer units also include a camera that photographs whoever provides the breath sample, along with GPS tracking that logs vehicle location.³ Having someone else blow for you is a criminal offense in most states, and the camera makes it easy to prove.

Who Has to Install One

A DUI or DWI conviction is the most common route. Thirty-nine states now mandate an interlock after a first offense, either as part of the sentence or as a condition of getting a restricted license back. That restricted license usually lets you drive during the interlock period, but only in a vehicle equipped with the device.¹ Repeat offenders face interlock requirements in virtually every state, often for longer periods.

A conviction isn’t the only trigger. Refusing a chemical test under implied consent laws often results in an administrative interlock requirement separate from any criminal case. Some states also permit voluntary installation: if your license has been suspended, installing an interlock early can shorten the wait before you’re allowed back on the road.

How Long You Keep It

Duration depends mostly on how serious the offense was and whether you’ve been convicted before. A first DUI typically carries a minimum of six months to one year. Second offenses commonly run one to two years, and third or subsequent convictions can mean three years or more depending on the state. High-BAC cases, where your blood alcohol was significantly above the legal limit at arrest, often carry longer periods even for a first conviction.

Those are minimums. Violations during the program frequently push the end date out. If the device records a failed breath test, a missed rolling retest, or evidence of tampering, most states add time, ranging from an additional 90 days in some states to a full extra year in others. Federal compliance-based removal requires that the final 40 percent of the installation period immediately before removal be completely clean.⁴ A single violation late in your program can reset the clock significantly.

What It Costs

You pay for the device yourself, and the costs stack up over months.

Installation is a one-time fee that varies by vehicle. Push-to-start ignitions and complex security wiring generally cost more than standard key-start models.
Monthly lease fees from major providers start in the mid-$50 range and run higher depending on the model and any state-mandated features like GPS or a camera.
Calibration appointments every 30 to 60 days typically cost around $20 on top of the monthly lease, though some providers bundle them into the rental price.
Violation resets, triggered when the device locks out after a failed test or tampering alert, run roughly $75 at a service center.
Removal fees range from about $50 to $130. Some states prohibit removal charges, but that’s the exception.

For a 12-month program, total cost usually lands between $1,000 and $2,500 once installation, monthly lease, calibration visits, and removal are all counted. Longer programs cost more. These figures don’t include indirect costs that catch people off guard: time off work for calibration appointments, higher car insurance premiums after a DUI, and potential towing charges if the device enters a permanent lockout.

Help If You Can’t Afford It

Roughly two-thirds of states offer some form of financial assistance for drivers who can’t cover interlock costs. Programs vary widely. Some states require vendors to provide reduced rates to qualifying participants; others maintain state-administered funds financed by DUI surcharges or tax revenue. Eligibility is usually tied to income relative to federal poverty guidelines, enrollment in public assistance, or a judicial finding of indigency similar to the standard for appointing a public defender.

Covered costs generally include installation and monthly monitoring. Violation reset fees are almost never covered, on the logic that non-compliance shouldn’t be subsidized. If you think you qualify, ask the court or your state’s motor vehicle agency about available programs before your installation appointment. About 10 percent or fewer of participants in states with these programs actually use them, which suggests many eligible people either don’t know the assistance exists or don’t apply.

Living With the Device

Installation typically takes one to three hours at an authorized provider. The technician wires the device in, then walks you through the blowing technique the unit requires. Take that training seriously; fumbling with the handset at 7 a.m. on a cold Monday is not the time to learn. You’ll receive a certificate of installation to file with your court or monitoring agency.

One of the more frustrating realities is that interlocks can register alcohol you didn’t drink. Common culprits include mouthwash and breath sprays, energy drinks, cough and cold medications, foods made with vanilla extract or yeast, baked goods containing alcohol such as rum cake, and even soy sauce or red wine vinegar. The fix is straightforward: rinse your mouth with water and wait at least 15 minutes after eating or drinking anything before you blow. The device measures deep lung air, not what’s sitting on your tongue, so a short wait usually clears the problem. A single failed test followed by a clean retest looks very different from a pattern of failures in your monitoring agency’s records.

Cold weather adds a wrinkle. Fuel cell sensors need to warm up before they can take an accurate reading, and modern devices are certified to reach operating temperature within about three minutes even in extreme cold. Some models let you pre-program up to three warm-up times per day so the unit is ready when you walk out the door. In below-freezing temperatures, condensation from your breath can freeze inside the mouthpiece; removing the mouthpiece after each drive and bringing it inside prevents ice buildup.

Calibration and Data Downloads

Every 30 to 60 days you bring the vehicle to your service center. The technician recalibrates the fuel cell, physically inspects the wiring for signs of tampering, and downloads the device’s internal memory. That log contains every breath test, every rolling retest, every engine start and stop, and, if your device has these features, photographs and GPS coordinates. The data goes directly to your monitoring agency, whether that’s a court, probation department, or DMV program. Missing a calibration appointment is a serious violation: most devices will lock out entirely if not serviced on schedule, and your state’s motor vehicle agency may suspend or revoke your driving privilege until you come back into compliance.³

Violations and Penalties

Violations generally fall into three categories: failed breath tests, missed or refused retests, and evidence of tampering or circumvention. Consequences escalate with severity and frequency.

A single failed startup test triggers a temporary lockout, typically five minutes initially and increasing with repeated failures up to 24 hours. You simply can’t start the car until the timer expires and you provide a clean sample. If the device isn’t brought in for scheduled service, or if violations accumulate, it can enter a permanent lockout where it won’t even prompt for a breath test. At that point you need either an override code from the service center or a tow truck.

The legal fallout is where things get expensive. Tampering with or circumventing an interlock, including unauthorized removal, is a misdemeanor in most states, carrying potential jail time and fines. More commonly, the penalty is an extension of your interlock requirement. The added time varies significantly: some states tack on six months per violation, others add up to a full year, and at least one state adds two years for a single tampering offense.⁵ Some states revoke the restricted license entirely, putting you back to square one with no legal way to drive.

Driving a vehicle that doesn’t have the required interlock is its own violation, separate from tampering. Getting pulled over in an unequipped car typically results in a citation, and a conviction leads to further suspension or revocation. This is where people trip up most often: borrowing a friend’s car or renting a vehicle without thinking about the restriction.

When You Don’t Need the Device in Every Vehicle

If your job requires you to drive a vehicle owned by your employer, you may be able to operate that vehicle without an interlock installed. Federal law explicitly permits this exception, and most states have adopted it in some form. The business cannot be owned or controlled by you, your employer must be notified in writing about the restriction, and the employer must grant written permission for you to drive the company vehicle without the device.⁴ That written documentation also goes to the court and your probation department. The exemption applies only to driving in the course and scope of employment, not to commuting in the company truck.

Two narrower federal exceptions exist: a physician certifying you cannot physically provide a deep lung breath sample, or no state-certified interlock provider operating within 100 miles of where you live.⁴ Neither comes up often, but they’re worth knowing about if either fits your situation.

Getting It Off

You can’t just walk into a shop and have the device taken out when your calendar date arrives. Removal requires written or electronic authorization from your monitoring agency confirming you’ve completed all program requirements. That usually means satisfying a violation-free period near the end of your term, commonly the last 90 consecutive days, though some states require a longer clean stretch. All outstanding fees must be current, and you’ll bring your ID, proof of insurance, and registration to the removal appointment.

A certified provider handles the removal, disconnects the device, and restores the modified wiring to its original condition, including soldering any severed connections. The provider then notifies your monitoring agency and the DMV that the requirement is complete, which clears the restriction from your driving record.

Attempting removal yourself, or having an unauthorized shop do it, is legally classified as tampering and carries the same penalties as any other circumvention offense. Even if you’re one day away from your scheduled removal date, jumping the gun can reset your timeline by months or years.

1
Insurance Institute for Highway Safety. Alcohol Interlock Laws by State
2
U.S. Department of Transportation – Volpe Center. Breath Alcohol Ignition Interlock Device (BAIID) Model Specifications
3
National Highway Traffic Safety Administration. Interlock Data Utilization
4
Office of the Law Revision Counsel. 23 USC 405 – National Priority Safety Programs
5
National Conference of State Legislatures. Penalties for Tampering with or Circumventing Ignition Interlock Devices