How an Interlock Device Works
An interlock device (also called an ignition interlock or IID) is a breathalyzer installed in your vehicle that tests your breath alcohol level before the engine will start. If the device detects alcohol above a set threshold — usually 0.02% blood alcohol content — the engine will not turn over. You blow into the mouthpiece, wait for the result, and if you pass, you can drive.
The device also requires rolling retests while you are driving. At random intervals after you start the car, a light or tone will alert you to provide another breath sample. If you fail a rolling retest, the device logs the violation and may honk the horn or flash the lights to alert you, though it will not shut off the engine when ready while you are moving. You must pull over safely and provide a passing sample at your next opportunity.
Every test — pass or fail — is recorded and stored in the device's memory. The data is downloaded by the monitoring company at regular intervals (usually monthly) and reported to the court or DMV. This creates a record of your driving and alcohol use that authorities can review.
Key Takeaways
- An interlock device is a breathalyzer installed in your car that prevents the engine from starting if it detects alcohol on your breath above the legal threshold.
- You must provide breath samples before starting the car and at random intervals while driving, with all results recorded and reported to the court or DMV.
- Interlock requirements are imposed by a court order or DMV decision, usually after a DUI or DWI conviction or a refusal to take a breath test.
- The length of time you must use an interlock device varies by state and by the severity of your offense, ranging from several months to several years.
- Tampering with, bypassing, or failing multiple tests on an interlock device can result in extended suspension, criminal charges, or both.
When a Court or DMV Orders an Interlock Device
An interlock device is ordered as a condition of license reinstatement or as part of a restricted driving permit after a DUI or DWI conviction. In some states, it is also required after you refuse to take a breath or blood test during a traffic stop — this is called an implied consent violation. The court or DMV decides whether an interlock is mandatory or optional, and for how long.
The length of the interlock requirement depends on your state and the details of your case. A first offense may require 3 to 12 months; a second offense within a certain period may require 1 to 3 years or longer. Some states impose interlock requirements even before you are convicted, as a condition of getting a restricted license while your case is pending.
You do not choose whether to use an interlock device — it is a legal requirement imposed by the court or DMV. If you refuse to install one when ordered, your license suspension continues and you cannot drive legally.
Finding an Approved Interlock Provider and Installation
Your state's DMV maintains a list of certified interlock providers — companies approved to install and monitor devices. You must use a provider on that list; devices from uncertified companies will not satisfy the legal requirement. Contact your state DMV or check its website for the approved vendor list in your area.
Once you choose a provider, you schedule an installation appointment. The technician will install the device on your vehicle's ignition system, which usually takes 1 to 2 hours. You will receive training on how to use it, what the alerts mean, and what happens if you fail a test. The provider will also explain the monthly monitoring process and any fees.
Installation costs vary by state and provider, typically ranging from $100 to $300. Monthly monitoring fees are usually $60 to $100. Some states cap these fees; others do not. You are responsible for paying all costs associated with the device.
What Happens If You Fail a Test or Miss an Appointment
A failed breath test — whether at startup or during a rolling retest — is recorded and reported to the monitoring company and the court or DMV. A single failed test does not automatically extend your interlock requirement, but multiple failures within a set period (often 30 days) can trigger a violation report.
If the court or DMV receives a violation report showing repeated failed tests, they may extend your interlock requirement by months or even years. In some cases, they may also revoke your restricted license and return you to a full suspension. A violation can also result in criminal charges if the court views it as evidence that you are driving under the influence.
You must also keep monthly or quarterly appointments with your interlock provider to have the device serviced and data downloaded. Missing an appointment is itself a violation and is reported to the court or DMV. If you miss an appointment, contact your provider when ready to reschedule; do not wait for a violation notice.
Tampering, Bypassing, and Other Violations
Tampering with or bypassing an interlock device is a criminal offense in all states. This includes disconnecting the device, having someone else blow into it for you, using a balloon or other object to simulate a breath sample, or attempting to disable or remove it. Even an unsuccessful attempt to bypass the device is illegal.
If you are caught tampering with an interlock device, you face criminal charges, which may include a misdemeanor or felony depending on your state and prior record. You will also face an automatic extension of your interlock requirement and likely a return to full license suspension. Your case may be referred to the prosecutor's office, and you may be required to appear in court.
Other violations include driving a vehicle without an interlock device installed when one is required, or driving a vehicle that belongs to someone else (whose vehicle does not have a device). Both are violations of your restricted license and can result in arrest and additional criminal charges.
Removing the Device and Ending the Requirement
Once you have completed the required interlock period without violations, you can request removal of the device. The process varies by state: some require you to file a petition with the court; others allow you to request removal directly from the DMV. Check your state's specific procedure — your interlock provider or the DMV can tell you what form to file and where to send it.
Before the device is removed, the monitoring company will read the final data and submit a compliance report to the court or DMV. If the report shows no violations and you have completed the full required period, the removal is usually approved. The provider will then schedule a removal appointment and uninstall the device from your vehicle.
Removal of the device does not automatically restore your full driving privileges. Your license may still be suspended or restricted in other ways depending on your case. Once the interlock requirement ends, check with your state DMV about what steps you need to take to restore your full license.
State Variations in Interlock Requirements
Interlock requirements differ significantly by state. Some states make interlock mandatory for all DUI convictions; others make it optional or require it only for repeat offenders or high blood alcohol levels. Some states allow judges discretion; others have fixed rules.
The length of the requirement also varies. A first DUI in one state might require 6 months of interlock; in another state it might be 12 months or longer. Some states count the interlock period as part of your overall suspension; others add it on top of the suspension period.
A few states have hardship exemptions that allow you to drive to work or medical appointments without an interlock device, though this is rare and usually requires a court petition. Most states require the device on any vehicle you operate, with no exceptions.
Because requirements vary widely, contact your state DMV or the court handling your case to learn the exact interlock rules that explore to you.
Frequently Asked Questions
Can I drive someone else's car if it doesn't have an interlock device?
No. If an interlock device is required as a condition of your license, you can only drive a vehicle that has an approved device installed. Driving any other vehicle — even a borrowed one — is a violation of your restricted license and can result in arrest and additional criminal charges.
What if I can't afford the installation and monthly fees?
Some states and counties offer fee waivers or reductions for low-income drivers, though these are not universal. Contact your state DMV or the court handling your case to ask about financial hardship options. Some interlock providers also offer payment plans. If you cannot pay, you cannot legally drive, and your suspension will continue.
Do I have to install the device on my own car, or can I use a family member's vehicle?
The device must be installed on the vehicle you will be driving. If you use multiple vehicles, each one must have an interlock device. If you only have access to a family member's vehicle, the device must be installed on that vehicle, and the family member must agree to it. You cannot legally drive a vehicle without the device if one is required.
What happens if I move to a different state while using an interlock device?
You must comply with the interlock requirement imposed by the state that ordered it, even if you move. Contact the court or DMV in the original state to learn whether your requirement can be transferred or modified. Moving does not end the requirement, and driving without the device in your new state is still a violation.
Can the interlock requirement be removed early?
In some states, you can petition the court for early removal if you have maintained a clean record with no violations for a set period (often 6 months). Other states do not allow early removal under any circumstances. Check your state's rules and the specific order from your court to see whether early removal is possible.