The breath test that counts in a Montana DUI case is given on a desktop instrument at a police station, sheriff's office, or detention center, not on the handheld device at the roadside. That instrument is the CMI Intoxilyzer 9000. This page explains how we know that, how the instrument works, and which certifications have to exist before a result is used against you. It is part of our Montana DUI Guide.
Which Instrument Montana Uses
The Administrative Rules of Montana do not name a brand or model. ARM 23.4.209 says all models of breath analysis instruments must be approved by the Forensic Science Division of the Department of Justice, and ARM 23.4.212(6) gives the Division the duty to select the primary instrument for use in the state.
The Division's choice shows up in its own documents. The Department of Justice publishes a Montana Senior Operator's Guide for the Intoxilyzer 9000, which describes it as "an infrared-based device," and a 2022 prosecutor training guide titled "Intoxilyzer 9000 Breath Test Admission Quick Guide." An earlier model, the Intoxilyzer 8000, was in use in 2013. The Montana Supreme Court described a test on that model in State v. Levanger, 2015 MT 83.
Those guides are undated or several years old, and agencies replace equipment. The reliable way to know what tested you is the Breath Analysis Report Form from your case, which prints the instrument and its serial number.
How the Instrument Measures Alcohol
The Intoxilyzer analyzes a sample of breath and reports an alcohol concentration. Under the rules, the sample it captures is "deep lung air," and the result is reported as grams of alcohol per 210 liters of breath (ARM 23.4.201(14), 23.4.212(8)). That is the same breath unit the DUI statute uses when it defines alcohol concentration (MCA § 61-8-1001(3)). Our article on how BAC is measured explains the relationship between breath and blood numbers.
The Division's guide does not describe the internal optics, and we do not go beyond it here. What it does describe in detail is the sequence the instrument runs every time.
What Happens During a Test
According to the Senior Operator's Guide, a subject test follows this order after the operator scans a permit card and enters the driver's information:
| Step | What the instrument does |
|---|---|
| Air blank | Clears the tubing and chamber of residual alcohol and sets the reading to 0.000. |
| Diagnostics | A self-check of the instrument's mechanical and analytical functions. |
| Calibration check | Reads a 0.080 dry gas standard. The reading must fall within 0.005 of 0.080. |
| First breath sample | The driver blows until the instrument signals that the sample requirements are met. |
| Repeat | The air blank, diagnostics, and calibration check run again before a second sample. |
| Second breath sample | The two samples must agree within 5% of their mean, or the instrument asks for a third. |
| Reported value | The final result is the lower of the two results that agree. |
The instrument will not start the sequence unless the operator's permit is current, and it asks the operator for the start time of a waiting period before the test. That waiting period has its own page: breath test procedure and the observation period.
The Errors It Is Built to Catch
The guide lists "exception messages" that stop a test. They are worth knowing because each one leaves a printed record:
- RFI Detect. Radio frequency interference, such as a cell phone or police radio near the instrument.
- Invalid Sample. A problem with the incoming breath. The guide says this could mean mouth alcohol is present, and the operator must start another waiting period before testing again.
- Ambient Fail. Something in the room air is interfering, such as a cleaning chemical or the subject standing too close.
- Improper Sample. The subject was allowed to blow at the wrong time.
- Diagnostic Fail. The self-check found a problem. The operator is told to keep the printout for the senior operator.
The instrument also locks itself in a "disabled" mode if its gas standard is empty or expired, if more than 31 days have passed since a valid field certification, or if the annual laboratory certification has lapsed.
The Certifications Behind a Result
| Requirement | What the rule says | Rule |
|---|---|---|
| Field certification | At least once every 31 days, by a senior operator, within plus or minus 10% of the target value | ARM 23.4.213(1) |
| Laboratory certification | Valid for 365 days. Standards must read within 5% or 0.005, whichever is greater | ARM 23.4.214(1) |
| Operator permit | No one may run a breath analysis without a current permit. Permits expire January 31 of the following year | ARM 23.4.218 |
| Model approval | Every instrument model must be approved by the Forensic Science Division | ARM 23.4.209, 23.4.212 |
The laboratory certification includes a review of the instrument's sensitivity for the detection of interfering substances (ARM 23.4.214(1)(d)). A field certification before a subject test, together with calibration checks during the test or a field certification afterward, creates an inference that the instrument was in proper working order (ARM 23.4.213(1)(i)). An inference is a starting point, not a final answer.
The Department of Justice's prosecutor guide lists the documents the State is expected to produce: the operator's certification, the field certification, the senior operator's certification, the annual certification, and the gas standard approval. It tells prosecutors to check that serial numbers match. A defense review checks the same thing.
Not the Handheld Roadside Device
The small device used at the roadside is a preliminary alcohol screening test, or PAST. It is covered by a separate rule. Its results are reported only as positive or negative, with 0.020 or more treated as positive (ARM 23.4.225(5), (6)). Taking it does not satisfy the obligation to take the evidentiary test (MCA § 61-8-1016(1)(d)). Our field sobriety tests page explains where it fits in a stop.
What This Means for Your Case
A breath result is a measurement made by a specific instrument, on a specific day, run by a specific operator. Each of those has a paper trail. Under MCA § 61-8-1018(1)(b)(i), a breath test report is admissible if the test was performed by a person certified by the Forensic Science Division, and under MCA § 61-8-1019(3) you are entitled to full information about the test. Reading those records against the rules is the first step in challenging a breath test, and it is one part of the wider review described in Montana DUI defenses.
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. Instrument details come from Montana Department of Justice publications and may change. It is not legal advice about your specific case.