What is an Ignition Interlock Device? How an IID Works
Written by BACtrack Editorial Team
Updated September 04, 2026
An ignition interlock device (IID) is a small breathalyzer, wired into a vehicle's ignition, that a driver may be required to install after a DUI, under an order from a court, a state DMV, or another program administrator [1]. Before the engine will start, the driver blows into a mouthpiece: a passing sample lets the car start, and a sample above the preset limit keeps the ignition locked.
Beyond that basic test, this article covers how the device works while you drive, who is required to use one, how an IID differs from a personal breathalyzer you might buy on your own, and what living with the device looks like day to day, including calibration and violations.
Requirements vary by state and change over time, so treat this as a general explanation rather than a statement of your specific requirement. It is not a cost guide and not legal advice, just a plain explanation of the device itself.
How Does an Ignition Interlock Device Work?
An IID uses a fuel-cell breath sensor connected to a control unit wired into the ignition. You blow into a handset before starting the car. A passing sample lets the engine start. A sample above the preset limit locks the ignition for a set time, and the attempt is recorded.
The device follows a repeating sequence:
- Start-up test. Before you can start the car, the device asks for a breath sample. The engine only starts if you pass.
- Rolling retests. While you drive, the device asks for more breath samples. Many programs schedule these at random times. This makes it harder for someone else to test for you and then let you drive after drinking.
- Logging. The device generally stores your results, including passes, fails, and skipped tests.
- Reporting to the authority. Stored results are typically sent to the monitoring authority, often the DMV. How often and how this reporting happens can vary by program.
A failed or missed rolling retest will not shut off the engine while you are driving. Turning off a moving car would be unsafe. Instead, the device logs the event, may sound a warning, and can lock you out for a set time. Whether an event counts as a reported violation depends on the program's rules. The preset limit on the device is a compliance rule. It does not tell you whether it's safe to drive.
Who Needs an Ignition Interlock Device?
Ignition interlock requirements exist in every US state, though the specifics differ significantly from state to state. According to the Insurance Institute for Highway Safety (IIHS), most states require some form of IID installation after an alcohol-impaired-driving conviction, but the requirement varies: some states mandate it for all offenders, some make it conditional on factors like BAC level or prior offenses, and a smaller number treat it as an optional or discretionary sentencing tool rather than a mandatory one. First-time offenders are included in many of these states, though not uniformly. As of its August 2026 update, IIHS lists seven states without a mandatory interlock law for all offenders: Indiana, Montana, Nevada, North Dakota, Rhode Island, South Dakota, and Wisconsin [6].
Research suggests interlocks are associated with lower reoffense and crash rates, though the size of the effect depends on the study and the comparison group used. A review of studies compiled by the CDC-backed Community Preventive Services Task Force found that among offenders who had an interlock installed, repeat impaired-driving re-arrest rates were a median of 67% lower than in comparison groups without an interlock, while the device was in place [4]. Separately, IIHS found that state laws requiring interlocks for all DUI offenders are associated with a 26% reduction in the share of drivers with a blood alcohol concentration of 0.08% or higher among drivers in fatal crashes, though this is an association from state-level law comparisons rather than a controlled study of individual outcomes [5].
In most cases, the requirement comes from a court or DMV order rather than a driver's own choice, though some state programs also allow voluntary installation. California is one clear example of how a state program works. There, most DUI convictions require an ignition interlock device under California Vehicle Code Section 23575 [2], and program lengths scale with the record: a first offense carries a minimum of 5 months, a second offense 12 months, a third offense 24 months, and a DUI conviction involving injury 12 to 48 months. Whatever state you are in, you will receive an official notification letter that states your exact requirement and your installation deadline, and it will tell you that you must use a certified provider.
For the exact length and deadline that apply to you, rely on your notification letter or your attorney, and for a fuller walkthrough of one state's process, see California's DMV rules for ignition interlocks.
Ignition Interlock Device vs. Personal Breathalyzer
An ignition interlock and a personal breathalyzer measure the same thing, breath alcohol, but they exist for opposite reasons. A personal breathalyzer is a device you buy to check your own number privately. An ignition interlock is a court- or DMV-ordered device that decides whether your vehicle will start and reports every reading to the state.
Three differences separate them cleanly: who orders it (you, versus the court or DMV), what it controls (nothing, versus the ignition), and who sees the results (only you, versus the monitoring authority). BACtrack makes personal breathalyzers as well as running the Drive interlock program, but the two are separate products for separate purposes.
|
Ignition Interlock Device |
Personal Breathalyzer |
|
|
Who orders it |
Court or DMV |
You |
|
What it controls |
Vehicle ignition |
Nothing |
|
Who typically sees results |
Monitoring authority |
Usually just you, unless shared through an app |
|
Installation |
Certified provider |
Self-purchased, no installation |
|
Purpose |
Compliance with a program requirement |
Personal reference |
BACtrack makes personal breathalyzers as well as running the Drive interlock program, and if a California program requirement applies to you, BACtrack Drive handles certified installation and monitoring.
Calibration and Violations
Once an ignition interlock is installed, it is not a set-and-forget device. It needs periodic servicing, and it records events that can count as violations.
Calibration keeps the readings accurate. Calibration intervals are set by state and provider, and they vary. In California, current rules require calibration at intervals not exceeding 60 days [3]. On your scheduled interval, you bring the vehicle to a service center so an authorized installer can recalibrate the device. It is a routine visit, not something you perform yourself. The same is true of installation and removal: all three are handled by the certified provider, never by the driver.
Missed service, missed tests, failed tests due to drunk driving, and any attempt to tamper with or circumvent the device can have consequences, though what counts as a violation and what follows it are set by your specific state or program. For a full breakdown of what counts as a violation, see what triggers an ignition interlock violation.
Getting an Ignition Interlock Device in California
If you are in California and need to get one installed, the process runs through a provider certified to install ignition interlock devices in the state, such as BACtrack Drive. You contact the provider with your driver's license number and your vehicle information. The provider verifies your eligibility with the DMV, schedules the installation at a service center, and prepares the required paperwork, including DMV form DL 920, which the installer submits to verify the device has been installed.
As of 2026, BACtrack Drive operates with a flat rate of $99 a month in California, covering monthly service, installation, and calibration visits (device removal is priced separately), using the ALCOLOCK AL-2500, and lists service centers in 24 California cities, from the Bay Area and Sacramento to Southern California [3]. BACtrack Drive also offers income-based discounts on ignition interlock cost for qualifying drivers, with eligibility checked when you get started.
Schedule an installation and BACtrack Drive will verify your eligibility with the DMV and get you set up.
Frequently Asked Questions
What is a restricted license with an ignition interlock device?
A restricted license is a limited driving privilege issued during a suspension, and what it allows and how you get one varies by state. In California, installing the device is not enough on its own: the DMV also requires verification of installation and confirmation of your eligibility before it issues a restricted license, and the restriction typically limits you to driving the equipped vehicle [7]. Requirements in other states may differ, so check your own notification letter or your state DMV for the specifics that apply to you.
Is a car breathalyzer the same thing as an ignition interlock device?
Yes, in common usage. "Car breathalyzer" is an informal name people use for an ignition interlock device (IID), and both terms refer to the same kind of device: one that measures breath alcohol concentration (BrAC) and controls whether the vehicle can start. The formal, DMV- and court-recognized term is "ignition interlock device." A personal breathalyzer you buy for your own use is a different product and is not connected to your vehicle.
What happens if I do not calibrate my ignition interlock device on time?
A missed calibration is treated as a program violation and can put your restricted license at risk. Calibration has to be done by the provider, not by you, so you schedule a service-center visit on the device's cycle. BACtrack Drive runs on a 56-day calibration schedule and reminds you before each visit is due.
Can someone else blow into my ignition interlock device for me?
Do not have another person provide a sample. Ignition interlock programs use anti-circumvention features and rolling retests intended to deter or detect substitution, and unauthorized assistance may be treated as a violation under the applicable program.
What does a lockout on an ignition interlock device mean?
A lockout generally means the IID prevents a normal vehicle start for a temporary period or until required service. The trigger, duration, and reset procedure depend on the device and state program. A failed rolling retest generally does not shut off the engine while the vehicle is moving.
Does a DUI, a DWI, or probation change my ignition interlock requirement?
DUI and DWI generally describe the same offense, driving under the influence of alcohol or drugs, and different states use different terms for it. Either offense can lead to an ignition interlock requirement, depending on your state's laws and the details of your case. If you are on probation, the terms of your probation may include the interlock requirement alongside other conditions, such as a DUI program or reporting requirements. Your notification letter, your court order, or your attorney can confirm which requirements apply to your specific case.
Is Smart Start the same as the device BACtrack Drive uses?
No. Smart Start is a separate ignition interlock device brand and provider, not the device BACtrack Drive installs. BACtrack Drive installs the ALCOLOCK AL-2500 for its California program. Both are examples of certified ignition interlock devices, but they are different manufacturers with their own installation networks and program terms. If your notification letter or court order does not specify an ignition interlock device program, you generally choose which certified service provider to use.
References
- CDC, "Ignition Interlocks" https://www.cdc.gov/impaired-driving/ignition-interlock/index.html
- California Vehicle Code Section 23575, FindLaw https://codes.findlaw.com/ca/vehicle-code/veh-sect-23575/
- BACtrack Drive, Ignition Interlock Service https://drive.bactrack.com/
- Community Preventive Services Task Force / CDC, "Alcohol-Impaired Driving: Ignition Interlocks," The Community Guide https://www.thecommunityguide.org/findings/motor-vehicle-injury-alcohol-impaired-driving-ignition-interlocks.html
- Insurance Institute for Highway Safety (IIHS), "State laws mandating interlocks for all DUI offenders save lives" https://www.iihs.org/news/detail/state-laws-mandating-interlocks-for-all-dui-offenders-save-lives
- Insurance Institute for Highway Safety (IIHS), "Alcohol and drugs" (state interlock requirements overview) https://www.iihs.org/topics/alcohol-and-drugs
- California DMV, "Statewide Ignition Interlock Device Pilot Program" https://www.dmv.ca.gov/portal/driver-education-and-safety/dmv-safety-guidelines-actions/driving-under-the-influence/statewide-ignition-interlock-device-pilot-program/
