Ignition interlock device laws require most drivers convicted of DUI to install a breathalyzer wired into their vehicle’s ignition that blocks the engine from starting if it detects alcohol on their breath. The fail point is set far below the 0.08 legal limit, usually between 0.02 and 0.025, so even one drink can keep the car off. The rules cover who installs one, for how long, what it costs each month, how you have to behave while it’s on your car, and what has to happen before it comes off. Getting any of those pieces wrong can add months or years to the time you spend driving on a restricted license.
Who Has to Install One, and for How Long
The order to install an interlock usually comes from two directions at once. Your state’s motor vehicle agency can suspend your license administratively right after a DUI arrest or a refusal to take a chemical test, then offer a restricted license conditioned on an interlock. Separately, the judge handling your criminal case can order the device as part of your sentence or probation. If both apply, you have to satisfy whichever runs longer.
Roughly 34 states and Washington, D.C. have all-offender interlock laws, which means the device is required for first-time DUI convictions, not just repeat or high-BAC cases. The federal government rewards these laws with highway safety grants: states that require interlocks for all convicted DUI offenders for at least 180 days qualify for additional funding under the Impaired Driving Countermeasures program.1eCFR. 23 CFR 1300.23 – Impaired Driving Countermeasures Grants
How long the device stays on depends on your record and your BAC at arrest:
- First offense: six months to one year. Courts sometimes waive it for low-BAC first offenders in states without an all-offender mandate.
- First offense with a BAC of 0.15 or higher: at least six months in most states, often longer.
- Second offense: one to two years.
- Third or subsequent offense: two to five years, with some states imposing lifetime requirements after a fourth conviction.
If you don’t install the device within the window your state allows, you lose all driving privileges until you comply. There is no grace period for driving unmonitored while you shop for a provider.
How the Device Works Day to Day
Before the engine will start, you blow into a mouthpiece attached to a small unit near the dashboard. The device compares your breath alcohol to its preset fail point, which most states set between 0.02 and 0.025, though a few go as low as 0.00 or as high as 0.05. Depending on your weight and how recently you drank, a single beer can be enough to lock you out.
Once you’re driving, the device prompts you for rolling retests at random intervals. You generally get two to three minutes to pull over safely and provide a sample. Miss the window or fail the retest, and the device logs the event and sets off an alarm, typically flashing the headlights or sounding the horn until you shut off the engine. It will not stop a running engine, but ignoring the alarm creates a recorded violation that your monitoring authority will see.
Around 40 states either require a camera on the interlock or let courts order one. The camera photographs whoever provides each breath sample, and the images upload to your monitor along with the breath data at your calibration appointments. The camera is there to stop someone else from blowing for you, and it can actually work in your favor if a household member fails a test while borrowing your car.
You cannot install the device yourself. Every state keeps a list of approved vendors, and installation has to go through one of them. After installation, you get a certificate that you file with the state to convert a full suspension into a restricted license. You then return for calibration every 30 to 60 days, depending on your state, so a technician can check the device’s accuracy and download your data. Missing a calibration counts as a violation.
What It Costs
A twelve-month interlock requirement generally runs between $1,000 and $2,500. The fees break down roughly like this:
- Installation: $50 to $200 one-time, with some providers running promotions as low as $0.
- Monthly lease and monitoring: $70 to $150, covering the hardware rental and data reporting.
- Calibration: sometimes included in the monthly fee, sometimes billed separately at $0 to $50 per visit.
- Removal: $50 to $100 one-time when the requirement ends.
Most drivers pay somewhere around $80 to $100 per month all-in. That figure doesn’t include high-risk auto insurance, which most states also require after a DUI. The SR-22 filing that proves you carry minimum liability coverage can push your premiums up significantly, and both the interlock and the insurance have to stay active at the same time for your restricted license to remain valid.
About 33 states offer financial assistance or reduced-fee programs for drivers who can’t afford the device. Eligibility is usually tied to income, often pegged to federal poverty guidelines or enrollment in programs like SNAP or Medicaid. You’ll generally need to submit financial documentation and, in some states, get a court finding of indigency. Installation and monthly monitoring fees are what these programs most commonly cover.
The Rules of a Restricted License
Your restricted license lets you drive only vehicles equipped with a certified interlock. That applies to every vehicle you operate, not just the one you own. Moving a neighbor’s car in the driveway counts. Any violation recorded on your device goes on your record no matter who was actually blowing, so lending your car to someone who fails a test creates a problem you’ll have to explain to your monitor.
Employer Vehicle Exemptions
About 21 states let you drive a company vehicle without an interlock during work hours under an employer exemption. The rules are tight: the vehicle has to be owned or leased by the employer, you cannot own or co-own the business, and you cannot use the company vehicle for personal errands or commuting. Your personal vehicle still needs the device. You typically have to carry a signed exemption form in the employer’s vehicle at all times, and the exemption usually runs one year and must be renewed. Self-employed drivers almost never qualify.
Moving to Another State
Crossing a state line does not end the requirement. The sentencing state expects you to finish the full term wherever you live. Interstate license compacts share DUI sanction information, so applying for a fresh license in a new state will not quietly erase the order. Before moving, arrange service with your interlock provider at a center in your new location and contact the new state’s motor vehicle department to find out how it will honor or add to your existing order. If the new state’s required period is longer, you may have to satisfy the longer term.
Avoiding False Positives
Interlocks measure alcohol in your breath, not your blood, and plenty of everyday products contain enough ethyl alcohol to trigger a fail even when you haven’t been drinking. The usual culprits are alcohol-based mouthwash, which can run up to 30% ethyl alcohol, along with some breath sprays and hand sanitizers. Less obvious triggers include certain cinnamon rolls, energy drinks, and mints or gums with trace ethyl alcohol.
The fix is simple. Rinse with plain water before every test, and wait at least a minute or two after eating or using any oral product. Mouth alcohol clears quickly because it’s not in your bloodstream. If you do fail a startup test and think residual mouth alcohol is the reason, wait 10 to 15 minutes, rinse again, and retest. Most devices build in a short temporary lockout after a failed attempt for exactly this reason. Passing on the follow-up usually resolves it, and a camera-equipped device helps because the photo shows you were the one testing.
Cold weather adds its own problem. In extreme cold, breath condensation can freeze in the mouthpiece, and the device itself needs time to warm up before it reads accurately. Most modern devices reach operating temperature within about three minutes even at very low temperatures. In a cold climate, build in a few extra minutes before your commute and keep the mouthpiece dry between uses.
Tampering and Non-Compliance Penalties
Tampering with an interlock is a separate criminal offense in nearly every state. That covers disconnecting the wiring, blocking the sensor, rigging a bypass, or asking someone else to blow for you. The person who blows for you is also committing a crime. Penalties commonly reach up to six months in jail and fines of several thousand dollars.
Driving a vehicle without the required interlock while your restricted license is active is treated like driving on a revoked license in most states. That usually means fresh criminal charges, immediate extension of your interlock period, and possible jail time. Many states add at least a full year to the monitoring requirement for each instance. Repeat violations can end in permanent revocation.
Courts also treat interlock violations as probation violations. If you’re on probation for the underlying DUI, a tampering charge or a run of failed tests can put you back in front of the judge facing the suspended portion of your original sentence. The device logs every successful test, every failure, every power disconnection, and every missed calibration appointment with a timestamp. Sensors are built to detect compressed air, filtered breath, and other non-human samples. Assume the device catches everything.
Getting the Device Removed
Running out the clock usually isn’t enough. About 33 states and Washington, D.C. use compliance-based removal, which means you have to put together a stretch of consecutive violation-free days at the end of your interlock period before the device can come off. The required clean period varies by state but commonly falls between 90 and 120 days.
Here is the trap. A violation during that final clean period doesn’t just add a few days. In many states it resets the clock. Some states restart only the final compliance window, others restart the entire interlock period from scratch. A single failed rolling retest in month eleven can send you back to month one. That reset is the strongest reason to take every test seriously from day one.
Early removal before your mandated period expires is possible in some jurisdictions but uncommon. You generally have to file a petition with the court, submit a certified compliance report from your provider showing a clean record, and attend a hearing. Judges typically grant early removal only when compliance is flawless and a significant portion of the original term has been served. In a compliance-based removal state, you’ll still need to show you completed the violation-free period even if the court agrees to shorten the overall term.
Medical Exemptions
Some people genuinely cannot produce a breath sample strong enough for the device to read. Conditions like COPD and emphysema can make it physically impossible to deliver the minimum breath volume. The federal interlock grant program lets states exempt individuals who have a physician’s written certification that they cannot provide a deep lung breath sample.1eCFR. 23 CFR 1300.23 – Impaired Driving Countermeasures Grants Some states instead reduce the required breath volume rather than waive the requirement, keeping monitoring in place while accommodating diminished lung capacity. The specifics depend on your state, but the starting point is always documentation from your doctor and approval from the court or motor vehicle agency.