In Montana, an ignition interlock violation falls into one of two groups. Driving a vehicle without the device, circumventing it, or helping a restricted driver get around it is a crime with up to 6 months in jail (MCA § 61-8-1024). A failed start, a lockout, or a missed service appointment is not a crime by itself, but it is recorded and reported, and it can cost you the probationary license. This page sorts out which is which. It is part of our Montana DUI Guide and follows on from ignition interlock and probationary licenses.
Two Kinds of Violation
The criminal statute is short. A person subject to an interlock restriction under MCA § 61-8-1010 may not operate a vehicle that is not equipped with a device. No one may knowingly assist a restricted person to start and operate the restricted person's vehicle. No one may knowingly circumvent the operation of a device. A conviction for any of these is punishable by a fine of not more than $500, imprisonment for not more than 6 months, or both (MCA § 61-8-1024).
Everything else, including what the device does when it detects alcohol, comes from the Department of Justice's rules for certified devices in ARM Title 23, chapter 3, subchapter 9. Those rules govern the device and the provider. They decide what gets logged and who is told.
What Each Event Leads To
| Event | What follows | Source |
|---|---|---|
| A failed start (breath sample over the set point) | The device will not start the vehicle and makes you wait: 5 minutes after the first failure, 10 after the second, and 30 for each later failure within 2 hours. The attempt is recorded. | ARM 23.3.957(13), (16) |
| Four failed initial starts within any 2 hours | A violation reset: the device must be brought in for an unscheduled service, and the provider reports it to the referring agency. | ARM 23.3.957(14)(a), 23.3.980(8) |
| Not servicing within 10 days of a violation reset | The device goes into lockout mode and the vehicle will not start. | ARM 23.3.957(15) |
| Missing a calibration appointment | Calibration and inspection are required at intervals of no more than 65 days. Failing to service on time triggers a violation reset. | ARM 23.3.980(1)(b), 23.3.957(14)(c) |
| Tampering with or circumventing the device | A crime: a fine of up to $500, up to 6 months in jail, or both. The provider must report it to the Motor Vehicle Division within 10 days. | MCA § 61-8-1024(3), (4); ARM 23.3.980(6)(e) |
| Driving a vehicle that has no device | The same crime and the same penalty. | MCA § 61-8-1024(1), (4) |
Failed Starts and the 0.025 Set Point
The device does not wait for 0.08. Under the department's rule, each device must be set to fail at 0.025 grams per 210 liters of breath unless the originating court mandates a different level (ARM 23.3.957(17)). A reading that would be far below the limit for a DUI charge still keeps the vehicle from starting.
After a failed start the device imposes a wait and must tell the driver how long. If the driver fails an initial start four times within any 2-hour period, the device requires a violation reset, meaning an unscheduled trip to the service center. If the vehicle is not serviced within 10 days of a violation reset, the device goes into lockout mode and will not start at all (ARM 23.3.957(13) to (15)).
People ask about mouthwash, medication, and food. The rule requires devices to purge residual alcohol before allowing another test, and the log is required to distinguish a device malfunction from a driver not meeting the requirements (ARM 23.3.957(11), (16)). If you believe a reading was wrong, write down what happened and when, and tell the provider promptly. A note made the same day is easier to rely on than a recollection weeks later.
Retests While Driving
The rules refer to "random breath tests" in addition to start-up tests, and they require the device to record the results of all tests and retests (ARM 23.3.957(6)(b), (16)). The rules we read do not list a missed retest, standing alone, as a trigger for a violation reset. They do define circumvention to include operating the interlock vehicle after the device detects excess breath alcohol (ARM 23.3.953(2)(h)).
In practical terms, a skipped or failed retest is logged, and continuing to drive after a failed one fits the rules' definition of circumvention. How a particular court responds to a missed retest depends on its order and on the judge.
Tampering and Circumvention
The rules define tampering as any act or attempt to disable or circumvent the device. Circumvention includes disconnecting any part of the device while the vehicle is in operation, altering it, using an air compressor or pump to supply a sample, supplying a sample from someone who is not the restricted driver, supplying an altered or filtered sample, and blocking a digital image identification device (ARM 23.3.953(2), (6)).
Approved devices must resist tampering and show evidence of it, and every lease must warn that tampering, circumventing, or misusing the device can lead to criminal prosecution (MCA § 61-8-1025(3)(e), (4)). The provider must report known tampering or circumvention to the Motor Vehicle Division within 10 days of determining that it occurred (ARM 23.3.980(6)(e)). There is one narrow exception in the criminal statute: starting the vehicle for safety or for mechanical repair of the device or the vehicle, when the restricted person does not drive it (MCA § 61-8-1024(5)).
Driving a Vehicle Without a Device
The restriction follows you, not one car. The statute makes it unlawful to operate "a vehicle" that is not equipped, and the rules require a certified device in any vehicle the restricted driver drives before the division issues the probationary license (ARM 23.3.980(1)(a)). We found no exception in either place for a borrowed car, a rental, or an employer's vehicle. Our guide to getting to work during a suspension discusses what that means on the job.
What a Violation Can Cost Beyond the Fine
- The probationary license. The Motor Vehicle Division's probationary license form says the license is withdrawn, and the full original suspension reinstated, if the holder fails to keep complying with its requirements or restrictions. A DUI court may revoke a license it authorized and impose the full suspension from that date (MCA § 61-5-231(2)).
- The sentence. An interlock condition is part of a court's sentence, so a violation can be brought back to the sentencing judge. See our guide to probation violations.
- A harsher next charge. A person arrested for DUI while under a court or department order to equip any vehicle with an interlock meets the definition of aggravated DUI (MCA § 61-8-1001(1)(b)).
- The device itself. A provider must remove the device if the account becomes 60 days past due, and must notify the referring agency and the division within 24 hours of any removal (ARM 23.3.980(4)).
If you are told you violated, ask for the data log before you answer for it. The log shows times, readings, and whether the device flagged a malfunction, and those details matter to how a court sees the event.
This page explains Montana law in general as of the 2025 Montana Code Annotated and the Administrative Rules of Montana in effect on October 9, 2026. It is not legal advice about your specific case.