Car Breathalyzer (Ignition Interlock): How It Works and What It Costs
A car breathalyzer, officially called an ignition interlock device (IID), is a small breath-testing unit wired into a car’s ignition. The driver must blow into it before the engine will start, and again at random intervals while driving. If it detects alcohol above a set limit, usually around 0.02 breath alcohol concentration, the car will not start and the result is recorded and reported. Most people get one because a court or state DMV requires it after a DUI or DWI.
Important: this article is general information, not legal advice. Interlock requirements, program lengths and fees are set by each state and your court order. Check with your state DMV, your interlock provider or a DUI attorney for the rules that apply to you.
How an Ignition Interlock Device Works
- Startup test: you turn the key or press start, and the device asks for a breath sample. You blow steadily into the mouthpiece for a few seconds, sometimes with a hum or a specific breathing pattern that makes it harder for someone else to blow for you.
- Result: if your breath alcohol is below the set limit, the car starts. If it is above, the car stays locked and you must wait before retesting.
- Rolling retests: after you start driving, the device asks for more samples at random times, commonly every 10 to 30 minutes or so. You get a few minutes to pull over safely before blowing.
- Recording and reporting: every test, failure, missed retest and any sign of tampering is logged. Many devices also take a photo of the person blowing. The data is downloaded at regular service visits and sent to your monitoring authority.
- Calibration visits: you return to the service center, often every 30 to 60 days, to download data and recalibrate the device.
A missed or failed rolling retest does not shut the engine off while you drive, which would be dangerous. Instead, it is logged, and some devices sound the horn or flash the lights until you stop the car and turn it off.
How Accurate Are Car Breathalyzers?
Court-ordered interlocks use fuel-cell sensors, the same basic technology as many police breath testers. A fuel cell reacts specifically to alcohol, which makes it far more accurate than the cheap semiconductor sensors in many keychain breathalyzers. Each approved device model must meet state certification requirements, and regular calibration visits keep readings accurate over time.
Most failed tests that are not caused by drinking come from mouth alcohol, such as mouthwash or food residue, rather than sensor error. Mouth alcohol fades within minutes, so a retest after rinsing with water usually clears it, and the logged pattern of a quick fail followed by a clean pass helps program monitors tell the difference. Very cold temperatures can slow the device’s warm-up but do not make it read alcohol that is not there.
How to Blow Into an Interlock Correctly
- Wait before testing: give it about 15 minutes after eating, drinking anything but water, smoking or using mouthwash.
- Rinse with water: a quick rinse clears food and drink residue.
- Take a normal breath, then blow steadily: exhale into the mouthpiece with a steady, continuous breath until the device signals it has a sample, usually several seconds.
- Follow the device’s pattern: many units require a hum or a brief inhale at the end; your installer will train you on your model.
- Seal your lips around the mouthpiece: air leaking around the sides can cause an “insufficient sample” error.
How Much Does a Car Breathalyzer Cost?
| Cost item | Typical range |
|---|---|
| Installation | $70–$150 |
| Monthly lease, monitoring and calibration | $60–$150 per month |
| Removal at the end of the program | $50–$150 |
| Missed appointment, lockout reset or violation fees | $25–$100+ each, varies by provider |
| Insurance filing (such as an SR-22) and higher premiums | Varies widely by state and insurer |
Over a typical 6 to 12 month requirement, many drivers pay roughly $500 to $2,000 in interlock costs alone, not counting fines, court fees or increased insurance. Some states offer reduced-cost programs for drivers who qualify based on income, so ask your provider or DMV. Many providers quote the lease as a daily rate, commonly around $2 to $4 a day, which works out to roughly $60 to $120 a month before calibration and other fees. The driver, not the state, pays these costs.
Who Has to Install an Ignition Interlock?
Every US state has an ignition interlock law, but the details vary widely. Depending on the state, an interlock may be required for:
- All DUI or DWI convictions, including many first offenses
- Repeat offenses or high blood alcohol levels
- Refusing a chemical test
- Getting a restricted license so you can drive to work or school during a suspension
Program lengths range from a few months to several years, and some states require a clean period with no violations before the device can come off. Organizations such as MADD support interlock requirements because research reviewed by NHTSA shows interlocks reduce repeat drunk driving while they are installed.
What Can Cause a Failed Test?
Interlocks detect alcohol in your breath, including from sources other than drinks. Common culprits include:
- Mouthwash and some breath sprays that contain alcohol
- Hand sanitizer used right before blowing
- Some foods and drinks, such as fermented foods, ripe fruit, certain baked goods, kombucha or energy drinks, can briefly leave alcohol in the mouth
- Cigarette smoke or vaping right before a test can interfere with the sensor
Most providers recommend waiting about 15 minutes after eating, drinking anything other than water, smoking or using mouthwash, and rinsing your mouth with water before you blow. If you fail, wait and retest; mouth alcohol usually clears quickly, while alcohol from drinking does not.
Living With an Interlock: Practical Tips
- Allow extra time: the startup test takes a minute or two, and warm-up can take longer in very cold weather.
- Keep your service appointments: missing a calibration visit can lock the car and be reported as a violation.
- Only you should drive the car, or anyone who does must be trained on the device, since every test is recorded.
- Never try to bypass it: tampering, disconnecting it or having someone else blow is illegal and is detected.
- Protect your battery: interlocks draw a little power; if the car sits for long periods, a battery maintainer helps, and because a weak battery can look like a device fault, it is worth knowing how to tell if your car battery is dead.
- Tell your mechanic before any repair that involves the battery or electrical system.
Voluntary Car Breathalyzers: Can You Put One on Your Own Car?
Yes. Anyone can install an interlock on a car they own without a court order. Parents use them on a teen’s car, people in recovery use them as a safeguard, and some employers fit them to fleet vehicles. Voluntary units are sold directly to consumers rather than leased through a state program, and the results go to you, or to a monitor you choose, instead of a court or DMV.
| Option | Typical cost | Notes |
|---|---|---|
| Buy a consumer interlock unit | About $400–$600 | Prices from consumer retailers in 2026 |
| Professional installation | About $130–$250 | Push-button-start cars usually cost more to wire |
| Monthly app or monitoring service | Optional, varies | Some units send alerts to a parent or monitor |
A voluntary device does not satisfy a court or DMV requirement. If you are ordered to install an interlock, it must be a state-approved model from a certified provider, with the required reporting.
Personal Breathalyzers vs Ignition Interlocks
The term “car breathalyzer” is also used for personal breathalyzers: keychain or plug-in devices, usually $20 to $200, that let you check your own breath alcohol level. They are a different product with a different purpose.
| Ignition interlock device | Personal breathalyzer | |
|---|---|---|
| Purpose | Legally required device that controls whether the car starts | Personal estimate of your alcohol level |
| Installed in the car | Yes, wired to the ignition | No, handheld or phone-connected |
| Accuracy | Fuel-cell sensor, professionally calibrated | Varies; cheaper semiconductor sensors are less accurate |
| Legally meaningful | Yes, results are reported | No |
| Cost | $60–$150 a month plus fees | $20–$200 one-time |
A personal breathalyzer reading below the legal limit does not mean you are safe to drive. Alcohol keeps absorbing into your blood after your last drink, devices can read low, and impairment starts well below the 0.08 legal limit in most states. If you have been drinking, the safest choice is not to drive.
Will New Cars Have Built-In Breathalyzers?
Federal law passed in 2021 directed NHTSA to develop a standard for technology that prevents drunk or impaired driving in new cars. Researchers and automakers are testing passive systems that detect alcohol through touch sensors or cabin air, rather than a device you blow into. As of 2026 there is no requirement in effect for new cars, and the timeline depends on the rulemaking process.
For more safety know-how, see our driving and safety guides.
Car Breathalyzer FAQ
Most devices are set to about 0.02 breath alcohol concentration, far below the 0.08 legal driving limit, though the exact setting depends on the state program.
Yes. Alcohol-based mouthwash, some breath sprays and certain foods can cause a positive reading. Wait about 15 minutes and rinse your mouth with water before testing.
It depends on your state and court order. Requirements range from a few months to several years, and some programs require a period with no violations before removal.
Anyone who drives the car must take the breath tests, and all results are logged under the program. Check your program rules before letting someone else drive.
Yes. Voluntary interlock units that stop the car from starting until you pass a breath test are sold to consumers for about $400 to $600, plus installation. They do not count toward a court-ordered interlock requirement.
Court-ordered interlocks usually cost about $60 to $150 a month for the lease, monitoring and calibration, on top of installation and removal fees.
