An interlock device test checks whether you can safely operate a vehicle after a DUI or DWI conviction

An interlock device test is not a written exam you study for. Instead, it measures your ability to blow into a machine that prevents your car from starting if it detects alcohol on your breath. If you have been convicted of driving under the influence, your state may require you to install this device in your vehicle and pass a test that shows you understand how to use it correctly.

The test itself is straightforward: you breathe into the device, it analyzes your breath alcohol content, and if the reading is below the legal limit (usually 0.02%), your car starts. If the reading is above that limit, the engine will not turn over. You are not being graded on knowledge or skill — you are demonstrating that you can operate the device as intended and that you understand the rules around it.

This is different from the written knowledge test you may have taken to get your license. That test covers traffic laws and road signs. The interlock test is a practical demonstration tied to a specific legal consequence of a DUI conviction.

Key Takeaways

  • An interlock device test requires you to blow into a breath-analysis machine to prove your breath alcohol content is below the legal limit before your car will start.
  • You take this test only if a court has ordered you to install an interlock device as part of a DUI or DWI sentence.
  • The device records every test you take, and your state's Department of Motor Vehicles reviews those records to monitor your compliance.
  • Passing the interlock test means staying sober while driving; failing it (blowing above the limit) triggers a lockout and is reported to the court.
  • The length of time you must use the device depends on your state's laws and the severity of your conviction, typically ranging from several months to several years.

When a court orders an interlock device

A DUI or DWI conviction often comes with a court order to install an interlock device in any vehicle you drive. The order specifies how long you must use it — this period is called your interlock monitoring period. In some states, you may have the option to install the device instead of losing your license entirely; in others, it is mandatory for all DUI convictions.

The court does not install the device for you. You must contact an interlock service provider (your state's DMV can give you a list of approved vendors) and schedule an installation appointment. You pay for the device, installation, and monthly monitoring fees — costs vary by state and provider but typically range from several hundred dollars upfront plus $60 to $100 per month.

Once installed, the device is connected to your vehicle's ignition system. Every time you start your car, you must provide a breath sample. The device logs the result, the date, and the time. Your state's DMV receives these records regularly and uses them to track whether you are complying with the court order.

How the interlock device test works

The first time you use the device, the service provider will walk you through the process. You insert your key or press the start button, and the device prompts you to blow into a tube. You exhale steadily for several seconds until the machine beeps. The device analyzes your breath alcohol content and displays the result on a small screen.

If your breath alcohol content is below the legal limit (usually 0.02%), the device allows your car to start normally. If it is above the limit, the device locks the ignition and will not allow the engine to turn over. The lockout is recorded in the device's memory and reported to your state's DMV.

Some devices also require a rolling retest — a second breath sample taken while you are driving, usually 5 to 15 minutes after you started the car. This prevents someone else from blowing into the device before you drive. If you fail a rolling retest, the device will not shut off your engine when ready (for safety reasons), but it will log the failure and alert authorities.

What happens if you fail the interlock test

Failing the interlock test means blowing above the legal limit. A single failure locks your ignition and is recorded. Multiple failures within a set period (often 30 days) can trigger serious consequences: your monitoring period may be extended, you may face additional fines, or your license may be suspended again.

The device also records circumvention attempts — actions like disconnecting the device, tampering with it, or asking someone else to blow into it for you. These are violations of your court order and can result in criminal charges on top of your original DUI sentence.

Your state's DMV reviews the device records regularly. If the records show repeated failures or violations, the court may revoke your driving privileges or impose additional penalties. The goal of the interlock program is to keep impaired drivers off the road; the device is both a safety tool and a monitoring tool.

How long you must use the device

The length of your interlock monitoring period depends on your state's laws and the details of your conviction. A first DUI offense typically requires 6 months to 1 year of interlock use. A second or subsequent offense usually requires 1 to 3 years. Some states allow you to reduce the monitoring period if you maintain a clean record — no failed tests, no violations — for a certain number of months.

You cannot straightforward remove the device when you think your time is up. Your service provider and your state's DMV must formally clear you. When your monitoring period ends, you schedule a removal appointment with the service provider. They disconnect the device, return it to their facility, and send a removal report to your state's DMV. Only then is your interlock requirement officially complete.

Preparing for and passing the interlock test

There is no study guide or practice test for the interlock device itself. The "test" is straightforward using the device correctly every time you drive. To pass, you need to stay sober. That is the entire requirement.

Before your first use, ask the service provider to explain the process step by step. Understand what the legal limit is in your state (usually 0.02%), how to blow into the device properly, and what rolling retests are. Write down the phone number for your service provider in case the device malfunctions or you have questions.

Keep a record of your own breath tests if possible — some devices allow you to request a printout of your records. This helps you track your compliance and can be useful if you ever need to dispute a failed test or prove your clean record to the court.

Interlock device vs. other DUI penalties

An interlock device is one of several penalties a court may impose after a DUI conviction. You might also face a suspended license, fines, mandatory alcohol education classes, probation, or jail time. The interlock device is often used as an alternative to a full license suspension — it allows you to drive to work, school, or medical appointments while proving you are not driving impaired.

Some states offer hardship licenses that let you drive only to essential places without an interlock device. Other states require the interlock device on all vehicles you operate, even if you also have a hardship license. The specific combination of penalties depends on your state's DUI laws, your criminal history, and the judge's decision.

Frequently Asked Questions

Can I drive someone else's car if I have an interlock device?

No. Your court order typically requires you to install the device in any vehicle you operate. If you drive a car without the device, you are violating the court order. Some states allow exceptions for vehicles owned by your employer or spouse, but you must get written permission from the court first.

What if the interlock device gives a false positive?

Devices can malfunction or give inaccurate readings due to calibration issues or interference. If you believe you failed a test unfairly, contact your service provider when ready and request a calibration check. Document the date, time, and circumstances. You can also request a hearing with the court to dispute the failed test if you have evidence of a device error.

Do I have to pay for the interlock device myself?

Yes. You are responsible for all costs: installation, monthly monitoring fees, and removal. Some service providers offer payment plans. In rare cases, a court may order the defendant to pay, but this is not common. The cost is part of the penalty for the DUI conviction.

What happens if I move to a different state while using an interlock device?

You must contact your original state's DMV and the court that ordered the device before you move. Some states have reciprocal agreements that allow you to transfer your monitoring to another state's program. Others require you to complete your full monitoring period in the original state, even if you move. Do not remove the device without explicit permission from the court.

Can I get my interlock monitoring period shortened?

Some states allow early removal if you maintain a perfect record — no failed tests, no violations, no missed appointments — for a set period, often 6 months. You must request early removal through the court or your state's DMV. The decision is up to the judge, and approval is not may provide. Check your state's specific rules with your service provider or DMV.