As lawyers with years of experience handling DUI cases, we’re often asked about ignition interlock devices, or car breathalyzers. Here, you’ll find the most common questions we’re asked—including how they work, the typical cost, and more.

What’s an ignition interlock device?

An ignition interlock device, or car breathalyzer, connects a vehicle’s ignition system to a breath-alcohol analyzer. To operate the device, a driver blows into a sensor that measures the alcohol in his or her breath and only allows the driver to start the vehicle if his or her breath-alcohol level is below the set point.

The device also takes photos to identify the person providing the breath sample, while recording the dates, times, test results, and photos of the person using the device. Drivers are subject to continuous monitoring while operating their vehicles. Any violations, for example failed tests or missed retests, are also recorded by the device.

What are Tennessee’s laws regarding ignition interlock devices?

As of July 2013, the state of Tennessee requires mandatory ignition interlock devices for anyone who is convicted of driving under the influence. This law also requires that devices have  cameras, so officials can tell who’s blowing into it. The law impacts drivers who have been arrested and convicted of drunk driving, and includes first-time DUI offenders with a breath test as low as .08 (House Bill 353; Senate Bill 670).

How accurate are these devices?

The National Highway Traffic Safety Administration (NHTSA) has established strict regulations for ignition interlock devices. Independent laboratories regularly test these devices to ensure they meet NHTSA’s requirements.

How is my driving information shared?

Typically, ignition interlock devices need to be serviced about every 60 days by the provider, who will download the information from your device and send it to the proper authorities. If you miss a service appointment, your device may prevent your car from starting.

How much does an ignition interlock device cost?

Drivers are responsible for all installation and service fees. The fee limitations set by the Tennessee Department of Safety state that ignition interlock device service providers must:

●     Charge installation fees of $150 or less.

●     Charge reasonable and customary fees that don’t exceed $100 per month to cover leasing, monitoring, and maintenance.

●     Charge reasonable and customary fees that don’t exceed $75 for the removal of devices.

●     Post these fee rates in a conspicuous area at the service provider’s office.


How long do I have to use the device on my car?

According to Tennessee law, an ignition interlock device must be in operation during the entire time period a restricted license is effectuated and must continue to be in operation six months after the license revocation period has expired. The license suspension periods are the following:

●     First offense: one year

●     Second offense: two years

●     Third offense: six years

●     Fourth or subsequent offense: eight years


What happens if I fail the breath test?

The ignition interlock device will prevent you from starting your car for several minutes after which you can attempt a second breath test. If you fail the second test, the device will require that you wait for a longer duration before retesting your breath-alcohol level.

Can a friend drive my car?

Not recommended.  If you allow someone else to drive your car, the device will take their picture and this could trigger a violation of your probation, resulting in jail time.  While it is technically legal for you to allow someone to drive your car, you may be accused of having that person blow into the interlock device so that you can drive drunk.

What happens if I don’t get the device installed on my car?

You may face serious consequences if you fail to install an ignition interlock that has been mandated by a court. Primarily your restricted license will not be approved and you will not be allowed to drive during the suspension.


There are two primary benefits to the interlock device.  First if you will most likely have no geographic restrictions on where you drive.  Before the interlock device restricted license allowed you to drive to and from work only.  Second, with an interlock device more people are eligible for a restricted license.  Before the interlock device only certain DUI offenders qualified for a restricted license.  Now virtually all DUI offenders should be able to get a restricted license.