How Can You Contest a DWI Charge Based on Breathalyzer Inaccuracies?
Sept. 9, 2025
Facing a DWI charge can be overwhelming, particularly when it’s based on a breath test result. The outcome of your case may hinge on whether that reading is accurate, yet breathalyzers are not infallible. From machine malfunctions and improper calibration to errors in administration, there are many factors that can produce false or misleading results.
At The Law Office of Aaron Fonseca, based in McAllen, Texas, I focus on helping clients challenge questionable evidence and protect their future. I understand DWI law and how breath test results are gathered, maintained, and presented in court. Here's more on how you can contest a DWI charge based on breathalyzer inaccuracies.
Recognizing Breathalyzer Limitations
Breathalyzers estimate blood alcohol concentration (BAC) by analyzing breath, but that estimate can be off. Interfering substances, like mouthwash, ketones from specific diets, or acetone from diabetes, can skew results. Environmental exposures, such as fumes from gasoline, solvents, or paint, might also affect the reading.
Switching to another angle, body factors matter too. A person’s body temperature, breathing pattern, lung capacity, and the timing of alcohol absorption can all change the result. That means a breath test doesn’t always reflect actual intoxication and shouldn’t be accepted at face value.
Questioning Calibration and Maintenance Records
Another starting point for DWI defense is method and machine reliability. Breathalyzers used in Texas, like the Intoxilyzer 5000 or 9000, must be calibrated and maintained by qualified personnel. If calibration or maintenance logs are missing or irregular, the device’s accuracy becomes suspect.
Moreover, disruptions or inconsistencies in upkeep can cast doubt on whether protocols were followed, making the breath results even more questionable under legal scrutiny. In some instances, these logs may reveal patterns of neglect or equipment failures that can be used to cast reasonable doubt in court.
Highlighting Procedural Errors by Law Enforcement
Even a properly functioning device can fail if misused. Texas law requires an officer to observe the driver for at least 15 minutes before administering a breath test to prevent contamination. Skipping this step, or not verifying that no belching or mouth alcohol occurred, can invalidate the result.
Other procedural mistakes, like using a handheld portable test instead of a certified station device, failing to certify the operator, or ignoring potential interference, can further undermine credibility. Proper procedure isn't just a technical requirement; it’s central to whether the results hold up under legal review.
Addressing Partition Ratio Variability
Breath tests assume a standard ratio between breath alcohol concentration and blood alcohol concentration, typically 2100:1. But individual bodies vary. A lower actual ratio means results may overstate the real BAC; a higher ratio may understate it.
That mismatch creates a solid line of reasoning for a DWI defense—showing that the presumptions baked into the device may not apply to every person. It also opens the door to arguing that the machine unfairly treated someone based on a one-size-fits-all assumption.
Considering Medical and Physiological Factors
Medical conditions or even certain diets can throw off a breath test. Issues like GERD, diabetes, asthma inhalers, or ketogenic diets can cause false positives or exaggerated BAC readings.
These conditions can result in molecules like ketones or stomach alcohol being detected as ethanol. For someone with those conditions, calling into question the breath reading becomes essential for health, and shouldn't be ignored when interference happens; the stakes are this high.
Highlighting Operator Error and Radio the testce
Sometimes the issue isn’t the machine, it’s how it’s used. If the officer fails to follow the test pr, making the mouthpiece improperly or not certified, raises red flags. Those errors can result in an unreliable reading.
Unexpected interference, like radio or cell phone signals, can also affect readings, even on devices that are designed to resist such problems. Though not always present, it can be a valid line of challenge when interference happens.
Pointing Out Timing and Absorption Phase Issues
The timing of when to successfully contest a breathalyzer result test matters, too. Breath tests may be done after the peak BAC has passed or before absorption is complete, making test results an imperfect snapshot of the driver’s state while operating a vehicle.
Retrograde extrapolation, models that estimate back to the actual time of driving, depend on widely varied assumptions. When inaccurately applied, these assumptions can misrepresent actual impairment. This is especially important in cases where the driver consumed alcohol shortly before being stopped.
Introducing Witnesses to Support DWI Defense
It’s often helpful to bring professional testimony—toxicologists, medical professionals, or individuals—into court to successfully contest a breathalyzer result. Technical knowledge can explain how pH factors may distort a reading.
These individuals can highlight flaws in calibration, medical predispositions, operator error, or faulty assumptions, and help build reasonable doubt around the breath test. Their insight can make a measurable difference when jurors try to make sense of scientific claims.
Comparing Breath Test With Other Evidence
Another way to reinforce a DWI defense is to compare the breath test outcome with other evidence. For example, field sobriety tests, dash or body-cam video, or personal behavior may not align with a high BAC reading.
Similarly, blood test results, when available, are more accurate and less prone to external interference. Divergence can support a challenge if the blood test contradicts the breath test. Discrepancies between types of evidence can give a judge or jury a reason to pause.
Mounting a Comprehensive Court Strategy
To challenge a breathalyzer result effectively, a DWI defense should:
Review calibration and maintenance logs.
Analyze officer conduct and the administration of the test.
Evaluate medical or dietary factors affecting BAC readings.
Compare breath results to other evidence, like blood tests or behavior.
Present testimony to explain technical or biological inconsistencies.
These elements form a multi-layered defense—not relying on one flaw, but creating consistent reasons to doubt the breath reading.
Preparing for Procedural Suppression Motions
You can file a motion to suppress the breath test result if significant issues emerge—such as missing maintenance logs, lack of certification, or protocol breaches. This makes the reading inadmissible, forcing the prosecution to rely on weaker supporting evidence.
Suppressing the evidence can increase the chance of a favorable outcome through reduced charges or acquittal. It can shift the balance of power in court and open the door to better negotiations or case dismissals.
Transitioning to Negotiation When Appropriate
Even when the breath test outcome can't be entirely suppressed, challenging its reliability can persuade prosecutors to lower charges. Armed with evidence of flaws and alternative explanations, your DWI defense may be enough to secure a plea to a lesser offense, such as reckless driving.
Challenging the prosecution's case can help reduce penalties and mitigate harsh sentencing. This can be especially important for those with prior records or professional licenses at risk.
A Breath Test Isn't the Final Word
A breath test result can feel final, but it shouldn’t be. There are numerous valid ways to question how it was collected, how the machine functioned, and how individual factors impacted its accuracy.
Using these methods as part of a vigorous DWI defense can transform a seemingly straightforward case into one full of reasonable doubt, supporting better results than expected when the reading looks unfavorable.
Experienced Criminal Defense
Challenging a DWI charge requires a coordinated approach that questions the breath test, highlights procedural flaws, and uses supporting evidence. At The Law Office of Aaron Fonseca, I can help individuals build a focused DWI defense that looks beyond the number on the machine. I proudly serve clients in Edinburg, McAllen, Mission, and the Rio Grande Valley of Texas. Call today.