How an ignition interlock device test works
An ignition interlock device (IID) is a breathalyzer installed in your car that measures your blood alcohol content before the engine will start. If you are required to use one — typically after a DUI or DWI conviction — you will need to pass a breath test each time you want to drive. The device does not start your car if it detects alcohol above a set limit, usually 0.02% BAC (blood alcohol content).
The test itself is straightforward: you blow into the device's mouthpiece for several seconds until it registers your breath sample. The whole process takes less than a minute. If your BAC is below the limit, the car starts normally. If it is above the limit, the engine will not turn over, and the device records the failed attempt.
Beyond the initial breath test, the device also requires rolling retests while you are driving. These happen randomly during your trip — you will hear a beep or see a light, and you have a set time (usually 5 to 10 minutes) to pull over safely and provide another breath sample. Failing a rolling retest or not providing one in time is recorded as a violation and reported to the court or monitoring authority.
Key Takeaways
- You blow into the mouthpiece before starting your car; if your BAC is below the set limit (usually 0.02%), the engine starts.
- The device will ask for random breath samples while you are driving, and you must provide one within the time window given.
- Failed tests, missed rolling retests, and attempts to tamper with the device are all recorded and reported to the court or your monitoring agency.
- You are responsible for keeping the device clean, scheduling monthly calibration checks, and paying the installation and monthly monitoring fees.
- The length of time you must use the device depends on your conviction and your state's laws, typically ranging from several months to several years.
What happens if you fail a breath test
A failed breath test — one that shows BAC above the legal limit — prevents your car from starting. The device logs the failure with a timestamp and stores the data. This information is downloaded during your monthly service appointment and reported to the court, your probation officer, or the monitoring company overseeing your case.
Repeated failures or a pattern of failed tests can result in serious consequences. Depending on your state and the terms of your sentence, violations may lead to extended IID requirements, additional fines, loss of driving privileges, or even jail time. A single failed test is not automatically a violation of your probation, but your monitoring authority will review the data and decide whether to take action.
If you know you will need to use the device, do not consume any alcohol for at least a few hours before driving. Some medications, mouthwashes, and foods can contain trace amounts of alcohol and may trigger a low-level reading, though a reading above 0.02% is unlikely from these sources alone.
Rolling retests and what they mean
While you are driving, the device will request a breath sample at random intervals. You will receive a warning — usually a beep, light, or vibration — and a countdown timer showing how long you have to provide the sample. You must pull over safely and blow into the device within that window, typically 5 to 10 minutes.
The purpose of rolling retests is to may support you are not drinking while driving and to prevent someone else from blowing into the device before you start the car. If you miss a rolling retest window, fail to provide a sample, or provide a sample above the limit, the device records it as a violation. Your vehicle may also honk the horn or flash the lights to alert you that a violation has been logged.
If you are driving in heavy traffic or in a situation where pulling over is unsafe, you can request a grace period in some cases, but this varies by device and jurisdiction. Always check your device manual or ask your service provider about their specific rules for rolling retests.
Monthly calibration and maintenance appointments
You are required to visit a service center monthly to have your device calibrated and serviced. During this appointment, the technician downloads all the data from your device — every breath test result, every rolling retest, every attempt to start the car — and prints a report. This report goes to the court, your probation officer, or the monitoring company managing your case.
The service center will also check that the device is working correctly, clean the mouthpiece, and replace any worn parts. You cannot skip these appointments; missing one is typically treated as a violation. If you move or cannot reach your regular service center, contact your monitoring authority when ready to find an approved location near you.
You will be charged a fee for installation (usually $100 to $300) and a monthly service fee (typically $50 to $100). Some jurisdictions offer fee waivers or reductions based on income, so ask your service provider or probation officer whether you may have access to.
Tampering, circumventing, and what counts as a violation
The device is designed to detect attempts to cheat it. Tampering includes trying to blow someone else's breath into the device, using a tube or bag to deliver air, or disconnecting the device from your car. The device has sensors that detect these attempts and log them as violations.
Other violations include failing to show up for a monthly service appointment, removing the device without permission, attempting to disable it, or accumulating too many failed breath tests. Some violations result in when ready consequences — such as a warrant for your arrest or an extension of your IID requirement — while others are reviewed by your monitoring authority before action is taken.
If you believe a test result was inaccurate, tell the service technician during your next appointment. They can note your concern in the report, and your probation officer or the court can review it. However, the device's reading is generally treated as reliable evidence, so contesting a result is difficult.
How long you will need to use the device
The length of your IID requirement depends on your conviction, your state's laws, and whether this is your first offense. First-time DUI offenders typically use the device for 6 months to 1 year. Repeat offenders or those with higher BAC readings at arrest may be required to use it for 2 to 3 years or longer.
Some states allow you to request early removal if you have maintained a clean record — no failed tests, no missed appointments, no violations — for a set period, often 6 months. You will need to file a petition with the court and may need to pay a fee for the removal process. Your probation officer or the service provider can tell you whether you are may be able to access and how to request early removal in your state.
Once your requirement ends, you will schedule a final appointment to have the device uninstalled. The service center will remove it, inspect your vehicle for any damage, and provide documentation that the device has been removed. Keep this documentation in case you are ever asked to prove the requirement is complete.
Frequently Asked Questions
Can someone else blow into the device for me?
No. The device is calibrated to your breath and has sensors designed to detect breath that does not match your baseline. Attempting to have someone else provide a sample is tampering and will be recorded as a violation. The consequences can include extended IID requirements, fines, or criminal charges.
What if I have a medical condition that affects my breath?
Certain conditions like diabetes, acid reflux, or respiratory issues can sometimes produce readings that seem high. If you believe a medical condition is affecting your results, tell your service technician and your probation officer. You may be able to request a blood test to verify your actual BAC, though this is not may provide and varies by jurisdiction.
Do I have to pay for the device myself?
Yes, you are responsible for installation and monthly service fees. Costs vary by location and provider, typically ranging from $100 to $300 for installation and $50 to $100 per month. Some courts or probation departments offer fee reductions for low-income individuals. Ask your service provider or probation officer whether you may have access to for a reduced fee.
What happens if I move to a different state?
You must contact your probation officer or monitoring authority before moving. They will help you locate an approved service center in your new state and transfer your case. Some states have reciprocal agreements that allow your IID requirement to continue; others may have different rules. Do not move without permission, as this can be treated as a violation.
Can I drive someone else's car instead of using the interlock?
No. Your court order or probation terms typically require that any vehicle you operate have an approved IID installed. Driving a car without the device while your requirement is active is a violation and can result in additional criminal charges, extended IID requirements, or jail time.