An ignition interlock device is a small breathalyzer wired into your vehicle’s starting system that keeps the engine from turning over if it detects alcohol on your breath. If a court or your state’s motor vehicle agency has ordered one, you’ll blow into it before every start, blow again at random intervals while you drive, pay a monthly lease fee for the length of your program, and bring the car in every few weeks for calibration. Here’s what to expect from the hardware, the rules, and the cost.
What It Is and How It Reads Your Breath
The handheld unit is about the size of a TV remote, connected to a control module a technician installs behind the dashboard and spliced into the ignition and starter wiring. You blow into a mouthpiece for several seconds. A fuel cell sensor inside reacts with any ethanol in your breath and generates an electrical current proportional to the alcohol concentration, which the device converts into a breath alcohol reading.
If the reading is below the programmed threshold, the starter engages normally. If it hits or exceeds the threshold, the vehicle won’t start and you’ll face a temporary lockout before you can try again. The federal model specifications from the National Highway Traffic Safety Administration set that threshold at 0.02 grams per deciliter, and most states program their devices at or near that level.1Federal Register. Model Specifications for Breath Alcohol Ignition Interlock Devices That’s well below the 0.08 legal driving limit, so even a single drink can prevent a start.
Many newer units also include a small camera that photographs whoever provides the sample, and a growing number of states require this feature so that someone else can’t blow for you.
Startup Tests and Rolling Retests
Once you’re on the road, the device requests additional breath samples at random intervals known as rolling retests. Under the NHTSA specifications, the first prompt appears within five to seven minutes of startup, with more prompts coming at random throughout the drive.1Federal Register. Model Specifications for Breath Alcohol Ignition Interlock Devices You have a short window to provide the sample. Miss it or fail it and the device logs a violation and sets off an alarm — horn honking, lights flashing — until you pull over and shut the engine off. The device never kills a running engine, because doing so at highway speed would be dangerous.
When a Court or DMV Will Order One
Interlock requirements come from two places: a judge’s order after a DUI conviction, or an administrative mandate from your state’s motor vehicle agency as a condition of getting your license back. Sometimes both apply.
Courts commonly order installation as part of probation or as an alternative to additional jail time, and even first-time offenders can be required to install one. The specifics depend on your state, but the direction has been toward broader mandates. As of 2026, all 50 states and the District of Columbia have interlock laws, and a majority require or allow installation for all DUI offenders, including first-timers.2National Conference of State Legislatures. State Ignition Interlock Laws
On the administrative side, most states require proof of installation before they’ll issue a restricted or hardship license during a suspension period. That lets you drive to work, school, or medical appointments while your full license stays suspended. Failing to keep the device installed and maintained for the whole mandated period can extend your restricted status or trigger a new suspension.
How Long You’ll Have It
Duration depends on your state and the severity of the offense. For a first DUI near 0.08, six months is a common minimum, though some states start at a full year. Second offenses typically carry one- to two-year requirements. Third and subsequent offenses escalate sharply, with some states requiring the device for five to ten years or even permanently.2National Conference of State Legislatures. State Ignition Interlock Laws
Higher blood alcohol levels at arrest often trigger longer periods even for a first offense. Someone caught at 0.15 or above may face the same duration as a repeat offender. The clock usually starts from the date of installation, not the date of conviction or sentencing, so delaying installation doesn’t earn you credit toward completion.
Getting It Installed
You need a state-certified provider. Only authorized vendors can install devices your court or motor vehicle agency will recognize, and only their compliance reports count toward your program. Your state’s motor vehicle agency website typically lists approved providers.
Bring a government-issued ID, your vehicle registration, and proof of insurance to the appointment, along with your court order or administrative notice showing the interlock requirement and any case numbers the provider needs for routing compliance data. The technician splices the control module into your ignition and starter wiring, usually a one- to three-hour job depending on the vehicle. You’ll then go through a training session on providing a proper breath sample, reading the device’s signals, and handling a lockout.
If other people regularly drive the car, their names need to go on file with the provider. You remain responsible for every reading the device records, regardless of who blew. A housemate blowing a 0.03 after a beer at dinner counts as your violation.
Employer Vehicle Exemption
If your job requires you to drive a company-owned vehicle, most states allow an employer vehicle exemption so the company truck or van doesn’t need a device installed. The conditions are strict. Your employer must be notified about your interlock restriction, you must carry written documentation of that notification while driving the work vehicle, and the exemption applies only during the course of employment, not for personal errands in the company car. It also doesn’t apply if you own or control the business, and it generally excludes school buses and large passenger vehicles.
What It Costs
Expect to pay roughly $100 to $200 for installation, plus a monthly lease and monitoring fee in the range of $60 to $100. Some providers also charge a separate calibration fee of $25 or more per visit. Removal at the end runs another $50 to $150. Over a 12-month program, total out-of-pocket costs typically land between $800 and $1,500, though it varies by provider and location.
A handful of states operate financial assistance programs for drivers who can’t afford the costs. Eligibility is usually based on income relative to federal poverty guidelines, and the help may cover a portion of monthly lease fees up to a capped amount. Check with your state’s motor vehicle agency to see whether a subsidy or indigency waiver is available.
Monitoring Appointments
You’ll return to the service center on a regular schedule, typically every 30 to 60 days, for calibration and data download. The technician recalibrates the fuel cell sensor using a controlled alcohol gas standard, inspects the wiring for signs of tampering, and downloads the device’s complete log. Every start, every retest, every violation goes to your monitoring authority. Missing a calibration appointment can itself trigger a violation and a device lockout.
Avoiding False Positives
Everyday products can trigger a failed test even when you haven’t been drinking. Alcohol-based mouthwash is the most common cause. Many popular brands contain enough ethanol to register if you rinse and blow within a few minutes. Breath sprays, cough syrups, liquid cold medications, and asthma inhalers can do the same.
Some foods also produce trace amounts of alcohol as they ferment or break down in your mouth. Ripe fruit, fresh bread, and kombucha are repeat offenders. Hand sanitizer fumes in a closed car can skew a test if you’ve just used a large amount.
The fix is simple. Wait at least 15 minutes after eating, drinking anything, or using any oral hygiene product before you blow. Rinsing with plain water before testing also helps. If you get an unexpected failure, stay calm. The device will offer a retest after the initial lockout, and by then whatever was in your mouth has dissipated. A single failed initial test followed by a clean retest is far less damaging to your compliance record than panicking and retesting immediately.
Lockouts: Temporary vs. Service
Lockouts come in two varieties, and they work very differently.
A temporary lockout happens after a failed startup test. You blow, the device reads above the threshold, and the vehicle won’t start for a set period, typically five minutes on the first failure, increasing to 10 or 15 minutes with consecutive failures. The cooldown gives mouth alcohol time to clear and discourages repeated attempts after drinking. Each failed test is logged and reported.
A service lockout is more serious. It triggers when the device detects tampering, when you miss too many rolling retests, or when you fail to show up for a calibration appointment within the allowed window. Once a service lockout activates, you can’t start the vehicle at all. The car has to be towed to a service center, or the provider has to issue a one-time override code, which not all providers will do and which generates its own report to the monitoring authority. Resolving a service lockout often involves additional fees and an automatic violation report that can extend your program.
Tampering Is Its Own Crime
State laws treat interlock tampering as a separate criminal offense, not just a program violation. Tampering includes disconnecting or rewiring the device, introducing compressed air or other external sources to fake a breath sample, and blocking or obscuring the camera. Having another person blow for you is also a crime, and in most states the person who helped can be charged as well.
Penalties vary by state but commonly include misdemeanor charges, additional fines, extended interlock periods, and potential jail time. The device is designed to detect many circumvention attempts. Fuel cell sensors distinguish between human breath and compressed air based on temperature, humidity, and flow characteristics. Cameras photograph whoever is providing the sample. Modern units log every electrical event, so even a brief disconnection shows up in the data download.
Driving a different vehicle to avoid the interlock is equally risky. If you’re caught operating any vehicle without an interlock while you’re under an interlock restriction, you face the same category of criminal charge plus the likelihood that your underlying DUI penalties get enhanced.
Moving or Traveling With the Device
Most states participate in the Interstate Driver’s License Compact, which means they share information about traffic offenses, license suspensions, and interlock requirements. Moving to a new state doesn’t cancel your interlock mandate from the original state. You’re generally expected to maintain the device for the full duration of the original order, even after establishing residency elsewhere.
The practical challenge is logistics. You need calibration appointments at regular intervals, and your original provider may not have service centers where you’re moving. Before relocating, coordinate with your provider about transferring to a partner location or a different approved vendor in the destination state. Verify with the new state’s motor vehicle agency that your restricted license will be recognized. If you’re on probation, your probation officer in the originating state needs to approve the move and may need to transfer supervision.
Short-term travel is simpler. You can generally drive your interlock-equipped vehicle across state lines without special permission, as long as you don’t miss any scheduled calibration appointments while you’re away.
Battery Drain
The interlock draws a small amount of power continuously, even when the vehicle is off, to maintain its internal clock and data log. The draw is modest, roughly a quarter of an amp, but it adds up if your car sits unused for more than a few days. A vehicle that sat fine for two weeks before installation might have a dead battery after the same period of inactivity.
Driving regularly is the simplest prevention. If you know the car will sit for an extended period, a battery maintainer (sometimes called a trickle charger) can keep the charge topped off without interfering with the device. Do not disconnect the interlock to preserve battery life. Even a momentary power loss gets logged and may be treated as a tampering event. If the battery does die, you’ll need it charged or replaced before the device will function again, and the outage will appear in your next data download.
Finishing the Program and Getting the Device Off
When your mandated period ends, removal isn’t automatic. You need a final calibration appointment where the provider performs a last data download and transmits it to your monitoring authority. That authority reviews your compliance record, verifying that you completed the full term, attended all calibration appointments, and didn’t accumulate unresolved violations. This review typically takes five to seven business days.
Do not have the device removed before you receive official written authorization. Unauthorized early removal counts as a violation that can reset your program clock entirely. Keep attending your regular calibration appointments right up until approval comes through, even if you’re past your original end date. Once removal is authorized, a technician disconnects the control module and restores your vehicle’s original wiring. You’ll also need to complete any remaining administrative steps with your motor vehicle agency, which may include paying a license reinstatement fee, to move from a restricted license back to full driving privileges.