Challenging the Breath Test: How a DWI Defense Attacks the Intoxilyzer 9000 Result

A breath number is evidence, not a verdict

Prosecutors present a breath result as an objective measurement of your blood alcohol concentration. It is not. A breath test is an indirect estimate produced by a machine, by an operator, under a protocol, and each of those can fail. Challenging it means treating the result as what it is, a piece of evidence with documented weaknesses, and attacking it methodically on the records, the procedure, and the science. For how the machine works, see the DWI breath test page; this page is about how the result gets dismantled.

Attack the records: calibration and maintenance

The Intoxilyzer 9000 must be calibrated, inspected, and quality-checked under the Texas Breath Alcohol Testing Program, and those records are discoverable. The defense subpoenas the maintenance logs, calibration results, and quality-assurance data and looks for gaps in maintenance, out-of-range calibration checks, or an instrument running on an expired certification. A machine that was not demonstrably working properly at the time of your test cannot be assumed to have produced a reliable number. See the motion to suppress.

Attack the procedure: the observation period and the operator

Texas protocol requires a continuous 15-minute observation period before the test, during which you must not belch, eat, drink, smoke, or regurgitate. This is one of the most common points of failure: officers who looked away, handled paperwork, or did not actually watch you for the full period have not satisfied the protocol, and a belch or silent reflux during that window can introduce mouth alcohol that inflates the reading.

The operator is the other procedural target. The officer running the test must be certified and must follow the steps exactly. Cross-examination on whether the observation period was truly maintained, whether the operator was properly certified, and whether the procedure was documented can undermine the reliability of the result, and creates a record that can be compared against the dashcam and bodycam video.

Attack the science: mouth alcohol and the partition ratio

The two scientific attacks are the most powerful and the least understood.

• Mouth alcohol. The machine is meant to read deep lung air, but alcohol lingering in the mouth, throat, or esophagus, from a belch, acid reflux, dental work, or an alcohol-containing product, can contaminate the sample and produce a falsely high result. The slope detector meant to catch this is not infallible.
• The partition ratio. The Intoxilyzer 9000 converts breath alcohol to blood alcohol using a fixed 2,100:1 ratio, a population average. Real ratios range from about 1,100:1 to 3,400:1, so a person with a lower ratio is overestimated, and someone whose true BAC is below 0.08 can read above it. In a case that turns on the 0.15 line, that difference can decide the level of the charge.

Attack the temperature, and the two-sample rule

Two more scientific attacks round out the breath challenge:

• Breath temperature. The machine assumes your breath leaves your lungs at 34 degrees Celsius, but normal body temperature is about 37. Research used in DWI litigation puts the effect at roughly 6.5 percent higher per degree Celsius above the assumption, so a person running warm, even slightly, can read meaningfully high. The instrument does not measure or correct for breath temperature.
• The two-sample rule. A valid Texas breath test requires two separate samples that must agree within 0.02 of each other. Samples that do not agree, or a test that captured only one usable sample, are a documented basis to question the result.

Why ACS-CHAL training changes the cross-examination

Most defense attorneys can argue in general that a breath test might be wrong. Challenging it at the level of the science is different. Managing Partner Deandra Grant was trained as both an operator and a maintenance technician of the Intoxilyzer, holds the ACS-CHAL Forensic Lawyer-Scientist designation and a Master’s Degree in Pharmaceutical Science, and devotes an entire chapter of her book, the Texas DWI Manual, to breath alcohol analysis. That lets the firm cross-examine the operator and the state’s experts on the partition ratio, the slope detector, the observation period, and the maintenance records with technical specificity, rather than in generalities. Partner Douglas Huff holds the same ACS-CHAL designation.

Frequently Asked Questions

Can a breath test result be challenged in Texas?

Yes. A breath result can be attacked on the calibration and maintenance records, the 15-minute observation period, operator certification and procedure, mouth alcohol, and the assumed partition ratio. It is evidence with documented weaknesses, not an automatic fact.

What is the 15-minute observation period, and why does it matter?

Texas protocol requires the officer to watch you continuously for 15 minutes before the test so no mouth alcohol contaminates the sample. If the officer did not truly maintain it, or you belched or had reflux, the result may be unreliable.

What is the partition ratio defense?

The Intoxilyzer 9000 assumes a 2,100:1 breath-to-blood ratio, but real ratios range from about 1,100:1 to 3,400:1. A person with a lower ratio is overestimated, so someone truly below the limit can read above it.

Can mouth alcohol affect a breath test?

Yes. Alcohol lingering in the mouth, throat, or esophagus from a belch, reflux, dental work, or an alcohol-containing product can contaminate the sample and inflate the result, and the machine’s slope detector does not always catch it.

What records can the defense get for a breath test?

The Intoxilyzer 9000’s calibration logs, maintenance records, and quality-assurance data are discoverable. Gaps, out-of-range checks, or an expired certification can undermine the reliability of the result.

Can body temperature affect a breath test?

Yes. The machine assumes breath at 34 degrees Celsius, but body temperature is about 37, and the reading rises roughly 6.5 percent per degree above that assumption. The instrument does not measure or correct for it, so a fever or simply running warm can inflate the number.

A Breath Result Is Not the Last Word.

The records, the procedure, and the science can all be challenged, and the difference can decide the level of your charge. Deandra Grant Law is trained on the Intoxilyzer itself. Call (214) 225-7117 for a free, confidential consultation.

Related Defenses Topics

• Challenging the Blood Test — Attacking blood evidence from the draw to the lab.
• Motion to Suppress — Getting unlawfully obtained evidence thrown out.
• Medical Conditions Defense — How GERD and diabetes inflate a breath result.
• DWI Defense Framework — How a strong defense works through the whole case.