A DUI interlock device is a breathalyzer wired into your car’s ignition that blocks the engine from starting if it detects alcohol on your breath. Thirty-one states and the District of Columbia now require one after every DUI conviction, including a first offense, and most remaining states mandate one for repeat offenders or high blood alcohol cases.1National Conference of State Legislatures. State Ignition Interlock Laws Expect to pay $60 to $120 a month to lease the unit and to keep it installed anywhere from six months to several years, depending on the severity of the offense.
How the Device Works
Before turning the key or pressing the start button, you blow a steady breath into a handheld unit mounted near the steering column. A fuel cell sensor detects ethanol and calculates the alcohol concentration in your breath. If the reading comes back above a preset limit, the ignition circuit will not complete and the engine stays off. That threshold is commonly set at 0.020 or 0.025 percent, well below the 0.08 legal driving limit.
Monitoring doesn’t stop once you’re on the road. The device prompts you for a rolling retest within five to fifteen minutes of starting the engine, then again at random intervals of roughly fifteen to thirty minutes. Fail a rolling retest or skip it, and the device triggers the horn and lights until you pull over and shut the engine off. It will not cut power mid-drive, because doing so would create a traffic hazard.
Many newer units include a built-in camera that photographs whoever provides each sample. Roughly half the states require camera-equipped devices to verify that the driver, not a passenger, is the one blowing.2National Highway Traffic Safety Administration. Alcohol Ignition Interlocks
Things That Can Cause a False Failure
The sensor is looking for ethanol, and plenty of everyday products contain it. Mouthwash is the most common culprit, with many popular brands containing 15 to 27 percent alcohol. Breath sprays, cough syrup, liquid cold medications, and kombucha can all leave enough residual mouth alcohol to fail a test. Even ripe fruit and fresh bread generate trace amounts as they ferment or break down.
Chemical fumes can also interfere. Hand sanitizer, perfume, and cologne contain alcohol-based solvents, and traces in the air near your face can skew a reading if you’ve just applied them. Some health conditions matter too. Acid reflux can push stomach gases into your mouth, and diabetes sometimes elevates acetone in the breath, which certain sensors misread as alcohol.
The practical fix is simple. Wait at least fifteen minutes after eating, drinking anything, or using any personal care product before providing a sample, and rinse your mouth with water first. If you fail a test and know you haven’t been drinking, most devices allow a retest after a short waiting period. A clean retest within about ten minutes usually keeps the initial failure from being reported as a violation.
Who Has to Install One
Whether you need an interlock depends on your state, the specifics of your offense, and sometimes a judge’s discretion. The broadest laws, now in place in thirty-one states and D.C., require an interlock for every DUI conviction, even a first offense at 0.08.1National Conference of State Legislatures. State Ignition Interlock Laws The remaining states generally mandate the device for repeat offenders or for first-timers who blew significantly above the legal limit, with trigger thresholds that range from 0.10 to 0.17 depending on the state.3Insurance Institute for Highway Safety. Alcohol Interlock Laws
Judges often have separate authority to order an interlock as a condition of probation, sometimes in exchange for reducing jail time. In that situation, the requirement flows from the court’s sentencing order rather than an automatic administrative rule.
Refusing a chemical test at the roadside creates its own path to an interlock. Every state has an implied consent law: by driving on public roads, you’ve agreed in advance to submit to testing if an officer has reasonable grounds to suspect impairment. Refusal triggers administrative penalties that apply whether or not you’re ever convicted, and in many states those include an automatic license suspension and a requirement to install an interlock before driving privileges come back. The suspension for refusing is often longer than the suspension for failing.
How Long the Requirement Lasts
The interlock period depends almost entirely on your offense history and your state’s statutory schedule. A standard first offense typically runs six months to one year. A second offense generally means one to three years. Third and subsequent offenses push the timeline to two to ten years, and a handful of states impose lifetime interlock requirements for the most serious repeat offenders.1National Conference of State Legislatures. State Ignition Interlock Laws
Those timelines assume you don’t trip any violations along the way. A failed breath test, a missed calibration appointment, or a tamper alert can reset the clock. Some states add a flat 120-day extension for each violation recorded during the final months of the requirement. Others restart the full mandatory period from the date of the violation. Every month of extension means another month of lease payments, calibration fees, and elevated insurance premiums.
Early Removal
A few states allow early removal if your compliance record is spotless. The general framework is that after a specified stretch of violation-free months, you can petition the licensing agency or request a hearing. Whether your state offers this option, and how long the clean period must be, depends on local law, so check with your state’s motor vehicle department before counting on getting out early.
Total Cost Breakdown
The device is leased, not purchased, and the costs stack across installation, monthly rental, calibration, removal, insurance, and administrative fees. Here’s what to expect:
- Installation: a technician wires the device into the ignition and electrical system. The labor fee typically runs $100 to $200, depending on the vehicle.
- Monthly lease: $60 to $120 for the hardware, data storage, and monitoring. This is the largest recurring expense and the one most affected by the length of your requirement.
- Calibration and service: the device must be recalibrated every 30 to 90 days depending on your state. Each visit costs roughly $25 to $50, and missing one counts as a violation.
- Removal: when the requirement ends, a technician removes the device and restores the original wiring. Removal fees typically range from $50 to $150.
- Insurance increase: this is the cost most people underestimate. Auto insurance premiums after a DUI conviction rise by an average of roughly 90 percent, which translates to about $2,000 or more per year in additional premiums for a typical driver.
- SR-22 filing: most states require you to carry an SR-22 certificate, a form your insurer files proving you meet minimum liability coverage. The filing fee itself is usually $25 to $50, but the real cost is the high-risk premium that comes with it.
- Administrative and reinstatement fees: the licensing agency charges its own fees to issue a restricted interlock license and eventually reinstate your full license, typically $50 to $200.
For a first offender with a six-month requirement, total out-of-pocket costs for the device alone (not counting insurance) generally land between $700 and $1,500. A second or third offender facing two or more years can easily spend $3,000 to $5,000 just on lease and calibration fees before insurance is factored in.
Getting the Device Installed
Installation happens at a licensed service center and usually takes one to two hours. You’ll need proof that you’re authorized or required to get the device, which means a copy of your court order or the notification letter from your state’s motor vehicle department. Bring identification, current vehicle registration, and proof of insurance. If your state requires SR-22 coverage, the SR-22 certificate must be filed with the licensing agency before or at the time of installation.
States maintain a list of approved interlock vendors. You must use a provider certified in your state, and the technician performing the installation must also be certified. Prices and service quality vary, so compare the monthly lease rate, calibration fees, and office locations before committing. A provider with a service center close to your home or workplace saves real time, since you’ll be visiting for calibration every one to three months for the length of the requirement.
At the appointment, the technician connects the device to your ignition system and runs through hands-on training. You’ll practice the correct blowing technique, learn the device’s signals, and walk through the rolling retest process. Pay close attention. An improperly provided sample looks the same to the device as a failed one, and repeated “insufficient sample” readings can trigger a lockout.
Ongoing Monitoring and Calibration
After installation, you return to the service center on a fixed schedule for calibration and data downloads. Frequency varies by state: about a third require visits every 30 days, roughly half set a 60-day interval, and a few allow up to 90 days between appointments. Some states require a 30-day initial visit followed by 60-day visits for the rest of the requirement.
At each visit, the technician verifies the sensor’s accuracy and downloads the device’s stored data, which includes every breath test result, the date and time of each engine start, any failed tests, missed retests, and tamper alerts. That data goes to whichever agency is monitoring your case: the court, a probation department, or the motor vehicle agency. If the device has a camera, photographs go with the report. Missing a scheduled calibration is treated as a violation and can extend your interlock period.
Employer Vehicle Exemptions
Many states offer a limited exemption that lets you drive an employer-owned vehicle for work without installing a second interlock in it. The vehicle must be owned, leased, or rented by the employer; you can only drive it during work hours for work-related purposes; and the employer must sign a declaration acknowledging your interlock restriction. You generally need to carry a copy of that declaration whenever you’re driving the exempt vehicle.
One catch trips people up. If you own or control the business, even partially, the exemption doesn’t apply. A sole proprietor, majority shareholder, or LLC member who is also the employer cannot write themselves an exemption letter. Personal vehicles used for commuting are never exempt regardless of your employment situation. The exemption also doesn’t cover commercial motor vehicles; driving a CMV on an interlock-restricted license is prohibited.
Violations, Tampering, and Penalties
The device records everything, and the data is reviewed at every calibration visit. Violations fall into two broad categories: test failures and tampering.
Test Failures and Lockouts
A breath test above the preset threshold triggers a temporary lockout of two to fifteen minutes, depending on how the device is programmed. After the lockout expires, you get another chance to blow a clean sample. Multiple consecutive failures can trigger a permanent lockout that requires a service center visit or a reset code to clear. Even a single confirmed failed test gets reported to the monitoring agency and can extend the requirement.
Skipping a rolling retest is treated just as seriously as failing one. When the device prompts you and you don’t provide a sample, it logs a missed test and starts the alarm sequence. If you’re in heavy traffic or can’t safely blow, pull over, provide the sample, and then continue driving.
Tampering and Circumvention
Attempting to disconnect, disable, or bypass the device is a criminal offense in every state with an interlock law. So is having someone else blow into it on your behalf. These offenses are typically charged as misdemeanors, with penalties that include fines ranging from $500 to $5,000, jail time of up to 18 months, and revocation of driving privileges. If you’re on probation for the underlying DUI, a tampering charge can also trigger a probation violation, which carries its own separate consequences including potential incarceration.
On the administrative side, tampering usually results in an extension of the interlock requirement or a full restart of the mandatory period from the date of the violation. Modern devices are designed to detect common bypass attempts. They log voltage interruptions suggesting someone unplugged the unit, and camera-equipped models flag mismatches between the person blowing and the registered driver.
Driving a Vehicle Without the Interlock
If your license has been restricted to interlock-equipped vehicles only, driving any vehicle that doesn’t have the device installed is a separate offense. Penalties vary by state but commonly include extension of the interlock period by a full year for a first violation and a complete license suspension for a second. Some states also add fines and jail time on top of the administrative consequences. The restriction applies to every vehicle you drive, not just your primary car. Borrow a friend’s vehicle or rent a car, and the interlock has to be installed in that vehicle too, or you cannot legally drive it.