What an interlock device test is and why it matters for your license
An interlock device test is a breath test you take before starting your car if you've been ordered to install an ignition interlock device. It's not part of the written knowledge test — it's a separate, real-world requirement that comes after a DUI or DWI conviction in most states. The device won't let your engine start unless you blow into it and register below the legal alcohol limit, usually 0.02% blood alcohol content.
This test happens every time you drive, not just once. You also have to pass rolling retests while the car is running — the device will prompt you to blow again at random intervals to make sure you haven't been drinking since you started driving. If you fail any test or try to tamper with the device, it logs the violation and typically alerts your state's monitoring program.
The interlock requirement is separate from getting your license back. You can't remove the device early just because you pass the written test or complete a DUI education program. The device stays in your car for the full period ordered by the court — usually 6 months to 3 years depending on your state and the severity of your offense.
Key Takeaways
- An interlock device test is a breath test you must pass before your car will start, and it's required by court order after a DUI or DWI conviction.
- The device tests you every time you drive and at random intervals while driving, and any failed test is recorded and reported to your state's monitoring program.
- You cannot remove the device early — it stays installed for the full period ordered by the court, which ranges from 6 months to several years depending on your state and offense.
- The cost of installation, monthly monitoring, and removal typically falls on you, and ranges from $100 to $300 per month depending on your state and provider.
- Passing the interlock test is not the same as passing the written knowledge test — both are required to restore your full driving privileges.
How the interlock device test works in practice
When you get in your car, the device displays a prompt asking you to blow into the mouthpiece. You take a deep breath and blow steadily for several seconds. The device analyzes your breath alcohol content and either allows the engine to start or locks it. If you're below the legal limit, you can start driving. If you're at or above the limit, the engine won't turn over and the device logs a failed test.
While you're driving, the device will randomly ask you to take a rolling retest — usually between 5 and 15 minutes after you started the car, and then at unpredictable intervals. You have a set amount of time (usually 3 to 5 minutes) to pull over safely and blow into the device again. If you don't take the test in time, or if you fail it, the device logs a violation. Depending on your state's rules, repeated violations can extend your interlock requirement or result in license suspension.
The device also records every test result, every failed attempt, and every time someone tries to tamper with it. Your monitoring company downloads this data regularly and reports it to your state's DMV or the court. You'll typically have a monthly or quarterly check-in where you bring the device in for data read and inspection, or the company does it remotely.
Who has to use an interlock device
Most states require an interlock device if you're convicted of DUI or DWI, especially if it's a first offense and you want to restore your driving privileges before the full license suspension period ends. Some states make it mandatory for all DUI convictions; others allow it as an option to shorten your suspension. A few states require it only for repeat offenses or high blood alcohol content readings.
The court or DMV will tell you whether an interlock is required in your case. You don't choose to install one — it's part of your sentence or the condition of getting a restricted license. If you're ordered to install one, you must do so within a set timeframe (usually 10 to 30 days) or face additional penalties.
Some states also require an interlock if you refused a breath or blood test after a DUI arrest, even if you weren't convicted. Check your court documents or contact your state's DMV to find out whether you're required to install one.
Interlock device costs and who pays
You pay for the interlock device yourself. Installation typically costs $100 to $300, and monthly monitoring and lease fees range from $60 to $150 depending on your state and the company. Removal at the end of your requirement period costs another $50 to $150. Over a full year, the total cost can reach $1,000 to $2,000 or more.
Some states have programs that reduce the cost for low-income drivers, and a few allow you to deduct the expense from your taxes as a crime victim restitution cost. Contact your state's DMV or the court that ordered the device to ask whether cost reduction programs exist in your area.
If you're required to install an interlock but can't afford it, you may be able to request a hardship waiver or extension from the court. However, most courts will not waive the requirement entirely — they may only delay it or reduce the monitoring period. Talk to your attorney or the prosecutor's office about your options.
What happens if you fail an interlock test
A single failed test doesn't automatically end your driving privileges, but it does get recorded. Your monitoring company reports it to your state's DMV or the court. If you have one or two failed tests over several months, you may receive a warning letter. If failures pile up — typically more than three in a month or more than five in a quarter — your state may suspend your license again or extend your interlock requirement.
Some states also impose fines for failed tests or violations. Check your court order or contact your monitoring company to find out what the consequences are in your state. The rules vary widely.
If you fail a test because you genuinely had no alcohol in your system but the device malfunctioned, you can request a calibration check or device replacement. Bring documentation of the failed test and any evidence that you weren't drinking — for example, a witness or a receipt showing you were somewhere else at the time. Your monitoring company can investigate and may clear the violation from your record.
Interlock requirements by state vary widely
Every state sets its own rules about who must use an interlock, how long they must use it, and what happens if they violate the requirement. Some states require it for all first-time DUI offenders; others only for repeat offenders or high BAC readings. Some states allow you to install one voluntarily to shorten your suspension; others make it mandatory.
The monitoring period also varies. A first offense might require 6 months of interlock use; a second offense might require 2 to 3 years. Some states count the time from installation; others count it from the date of your conviction or arrest.
Because the rules are so different, you need to check your specific state's DMV website or your court order to understand what you're required to do. The monitoring company you choose will also explain the rules for your state when you install the device.
How an interlock device affects your license restoration
Installing and passing interlock tests is one step toward getting your full driving privileges back, but it's not the only step. You also typically have to pass the written knowledge test, pay reinstatement fees, and complete a DUI education program. The interlock device itself doesn't restore your license — it's a condition you must meet while your license is suspended or restricted.
Once you've completed your interlock requirement (usually 6 months to 3 years), you can request removal. Your monitoring company will schedule an appointment to uninstall the device. After removal, you'll need to submit proof of completion to your state's DMV to restore your full driving privileges. Some states require you to pass the written knowledge test again before full restoration; others don't.
Even after the interlock is removed and your license is fully restored, the DUI conviction stays on your driving record. This affects your insurance rates and your ability to drive in other states for several years.
Frequently Asked Questions
Can I remove the interlock device early if I pass all my tests?
No. The device must stay installed for the full period ordered by the court, even if you pass every test. Early removal is only possible if you petition the court and the judge agrees to modify your sentence, which is rare. Contact the court that ordered the device if you believe there are exceptional circumstances.
What if I can't blow hard enough to register a reading on the device?
The device is designed to detect weak or fake blows. If you have a medical condition that makes it hard to blow, tell your monitoring company and bring documentation from a doctor. They may be able to adjust the device settings or provide a different type of test. Repeated failed attempts to blow will be logged as violations.
Do I have to take the interlock test if someone else is driving my car?
Yes. The device is installed in your car, so anyone who drives it must blow into it before starting the engine. If a family member or friend refuses or fails the test, the engine won't start. This is one reason many people with interlock requirements use a second vehicle or rely on other transportation while the device is installed.
Will the interlock device show up on my driving record?
The interlock requirement itself is part of your court sentence and will appear on your driving record. Insurance companies can see it, and it will affect your rates. However, the device itself is not visible to other drivers — it's installed inside your car and not obvious from the outside.
What happens if I move to a different state while on interlock?
You must notify your monitoring company and your original state's DMV before you move. Some states have agreements to recognize interlock requirements from other states; others don't. You may be required to transfer your device to a monitoring company in your new state, or you may have to complete your requirement in your original state even if you've moved. Contact both your current and new state's DMV to find out what applies to you.