The interlock test checks whether you can drive safely after an alcohol-related conviction

An interlock driver license test is not a written exam or a road test. It is a breath alcohol monitoring device installed in your vehicle that measures your blood alcohol content (BAC) before the engine will start. If you have been convicted of driving under the influence (DUI) or driving while intoxicated (DWI), a court or your state's Department of Motor Vehicles may require you to install one as a condition of getting your license back or keeping it.

The device itself looks like a small breathalyzer mounted on your dashboard or steering column. You blow into it before starting the car. If your BAC is below the legal limit set by your state (usually 0.02% or lower), the engine starts normally. If your BAC is at or above that limit, the car will not start, and the device records the failed attempt. Some devices also require rolling retests — additional breath samples while you are driving — to prevent someone else from blowing into the device before you start.

This is different from a standard road test, which evaluates your driving skills. The interlock test is purely about whether alcohol is in your system at the moment you try to drive.

Key Takeaways

  • An interlock device measures your breath alcohol content before your car will start, not your driving ability.
  • You must blow into the device each time you want to drive, and it records all attempts — both passing and failing.
  • The device may require rolling retests while you are driving to confirm you have not consumed alcohol during the trip.
  • Installation, monthly monitoring, and removal are your responsibility to pay for, with costs varying by state and service provider.
  • How long you must use the device depends on your conviction, your state's laws, and sometimes your BAC at the time of arrest.

How the device works during a typical drive

When you get in your car, you turn the key or press the start button. The interlock device activates and displays a prompt on its small screen. You place your mouth on the mouthpiece and blow steadily for several seconds — the device needs a full breath sample to measure accurately.

The screen shows your BAC result within seconds. If it is below the state-set threshold (commonly 0.02%), the device beeps or lights up to signal that you can start the engine. If your BAC is above the limit, the device locks the ignition and may sound an alarm. The failed attempt is logged with a timestamp, and you cannot start the car until you wait and try again.

Once you are driving, the device may ask for a rolling retest — another breath sample while the car is in motion. This prevents someone from blowing into the device before you start and then you driving after drinking. If you fail a rolling retest or do not provide one when prompted, the device typically logs the violation and may trigger the horn or lights to alert you to pull over safely.

What the device records and who sees it

Every time you use the interlock device, it creates a record: the date, time, BAC result, and whether you passed or failed. The device also records if you tried to tamper with it, disconnected it, or missed a scheduled service appointment. This data is stored on the device itself and downloaded during your monthly service visits.

Your service provider — the company that installed and maintains the device — sends these records to the court or your state's DMV on a regular schedule, usually monthly. A single failed test does not automatically revoke your license, but a pattern of failures or tampering can result in license suspension or extension of your interlock requirement. Some states allow one or two failed tests before taking action; others have stricter policies.

You have the right to see your own records. If you believe the device malfunctioned or recorded a false positive, you can request a calibration check or ask your service provider to document the issue. Keep records of any mechanical problems or unusual readings so you have evidence if you need to dispute a violation.

Installation, maintenance, and cost

Once a court orders you to install an interlock device, you must contact an approved service provider in your state — your court order will list which companies are authorized. You schedule an appointment, and a technician installs the device, usually in under an hour. Installation costs range widely by state and provider, typically between $100 and $300.

After installation, you must visit the service provider monthly for a calibration check and data read. This appointment usually takes 15 to 30 minutes. Monthly service fees typically range from $60 to $100, though some states cap the cost or offer reduced fees based on income. You pay these costs out of pocket — they are not covered by insurance or public programs.

When your interlock requirement ends (based on your court order or state law), you must schedule a removal appointment. The technician uninstalls the device, and you pay a removal fee, usually $50 to $150. If you do not remove it when ordered, you may face additional fines or license suspension.

How long you must use an interlock device

The length of your interlock requirement depends on your state's laws, the severity of your conviction, and sometimes your BAC at the time of arrest. A first DUI conviction typically requires 6 to 12 months of interlock use. A second conviction usually requires 12 to 24 months. A third or subsequent conviction may require 24 months or longer.

Some states reduce your interlock period if you maintain a clean record — no failed tests, no missed appointments, no tampering — for a set number of months. Other states have fixed periods with no reduction option. A few states allow you to request early removal if you meet specific conditions, such as completing an alcohol education program or maintaining a perfect record for a certain period.

Your court order will specify your exact end date or the conditions you must meet to have the device removed. Keep this order and bring it to your final service appointment so the technician can verify that your requirement has ended.

What happens if you fail a test or tamper with the device

A single failed breath test does not automatically end your driving privileges, but it is recorded and reported to the court or DMV. If you have occasional failed tests, you may receive a warning. If failures become frequent or show a pattern, your license can be suspended or your interlock requirement extended.

Tampering with the device — disconnecting it, trying to bypass it, or asking someone else to blow into it for you — is a serious violation. The device logs tampering attempts, and they are reported when ready. Tampering can result in criminal charges, license revocation, and extension of your interlock requirement by months or years, depending on your state.

If you believe a failed test was a false positive — caused by mouthwash, certain foods, or a device malfunction — report it to your service provider when ready and request a calibration check. Document what you consumed or did before the test. Some states allow you to dispute results if you can show the device was not functioning correctly.

Interlock requirements vary significantly by state

Every state has different rules for who must use an interlock device, for how long, and at what cost. Some states require interlock for all DUI convictions; others only for repeat offenders or high BAC cases. Some states allow you to request interlock as an alternative to license suspension; others make it mandatory.

A few states have ignition interlock programs that allow you to drive to work, school, or treatment while your license is suspended, as long as you use the interlock device. Other states require interlock only after your suspension period ends. The cost-sharing rules also vary — some states subsidize low-income drivers; others do not.

Your court order or DMV notice will specify your state's requirements for your situation. If you are unclear about your obligations, contact your local DMV or the court that issued your order. Many states also have a state interlock program office that can answer questions about approved providers and requirements.

Frequently Asked Questions

Can I drive someone else's car instead of using the interlock device?

No. Your interlock requirement is tied to you, not to a specific vehicle. If you are required to use an interlock device, you must install it in any vehicle you drive regularly. Driving a car without the device while under an interlock order is a violation and can result in criminal charges and license suspension.

What if the interlock device says I failed when I did not drink?

Report it to your service provider when ready and request a calibration check. Certain foods (like ripe fruit or vinegar), mouthwash, breath spray, and some medications can produce false positives. Document what you consumed before the test. If the device is found to be malfunctioning, the service provider can document this and report it to the court, which may help you dispute the violation.

Do I have to pay for the interlock device myself?

Yes, in most states you pay for installation, monthly service, and removal out of pocket. Some states offer reduced fees or waivers for low-income drivers, and a few states subsidize part of the cost. Contact your state's DMV or interlock program office to ask whether cost information is available in your area.

What happens if I miss a monthly service appointment?

Missing an appointment is recorded as a violation and reported to the court or DMV. Depending on your state, one missed appointment may trigger a warning, but repeated missed appointments can result in license suspension or extension of your interlock requirement. Schedule appointments in advance and set reminders to avoid missing them.

Can the interlock requirement be removed early?

Some states allow early removal if you maintain a perfect record — no failed tests, no missed appointments, no tampering — for a set period, often 6 months. Other states have fixed periods with no early removal option. Check your court order or contact your state's DMV to learn whether early removal is possible in your situation.