A number on a breath machine can feel like the end of the argument. You blew a 0.10, the officer says the case is closed, and you assume there is nothing to fight. I hear that assumption from worried people almost every week, and it is wrong more often than you would think.
Breath testing machines are not lie detectors, and they are not the gold standard that TV makes them out to be. They are electronic instruments that estimate the alcohol in your blood by measuring the alcohol in a puff of your breath, and every step in that process can go wrong. When it does, sober or barely-over people get arrested and charged based on a false number. Here is how those errors happen and why the reading is not the last word.
What a Breath Machine Actually Does
Start with what the device is really doing. It does not measure the alcohol in your blood directly. It measures alcohol in your deep lung air, then multiplies by a conversion factor to estimate your blood alcohol concentration. That conversion assumes your body matches an average ratio, often cited as 2100 to 1, between alcohol in the blood and alcohol in the breath.
The trouble is that this ratio is not the same for everyone. It varies from person to person and even within the same person over the course of a day, depending on body temperature, breathing pattern, and other factors. If your real ratio is different from the machine's assumption, your reading can come out higher than your actual blood alcohol level. You did nothing wrong and the machine still overshoots. This is one of the core issues we raise in a BAC defense.
Mouth Alcohol: A Common False Positive
Breath machines are built to sample deep lung air, not alcohol lingering in your mouth. When alcohol is present in the mouth or throat, the device can read it as if it came from your lungs and report a falsely high number. This is the mouth alcohol problem, and there are several everyday ways it happens.
A recent sip of alcohol, a burp, acid reflux, or GERD can bring alcohol vapor from the stomach up into the mouth right before you blow. So can a belch. Even mouthwash, breath spray, or cough medicine containing alcohol can do it. Something stuck in your teeth that traps alcohol, like food or dental work, can hold it too.
This is exactly why the officer is supposed to watch you continuously for about 15 minutes before the test, to make sure you do not burp, regurgitate, or put anything in your mouth. When that observation period is cut short or ignored, which happens, a mouth alcohol reading can slip through and inflate your result. That failure to follow procedure is something we look for in every case.
Medical Conditions That Fool the Machine
Your body chemistry can produce a high reading even when you have had little or nothing to drink. People with diabetes or those on low-carb or fasting diets can produce acetone and other ketones on their breath, and some machines can misread those compounds as alcohol. GERD and chronic acid reflux, as mentioned, can push stomach contents up into the airway. Certain other medical conditions affect the result too.
If a doctor could explain your reading better than a bartender could, that is a defense worth developing. We have a whole discussion of this on our FAQ about whether a medical condition can cause a false high BAC reading.
Calibration, Maintenance, and Machine Error
A breath machine is only as trustworthy as its upkeep. These instruments have to be calibrated and maintained on a regular schedule, and the agency is supposed to keep records proving it was done. When we get a DUI case, one of the first things we request is the maintenance and calibration history for the specific device that tested you.
We are looking for missed calibration dates, prior malfunctions, repair logs, and any accuracy check that came back out of tolerance. A machine that failed its own accuracy checks around the time it tested you is a serious problem for the prosecution. Radio frequency interference, electrical issues, and simple software or sensor faults can also throw off a reading. The number on the screen looks precise, but precision is not the same as accuracy.
Operator Error
Even a perfectly maintained machine gives bad results in untrained or careless hands. The officer has to follow a specific protocol: observe you for the required period, have you provide a proper deep-lung breath sample, run the test correctly, and record everything. Rushing the observation window, coaching you to blow in a way that skews the sample, or failing to note that you burped are all mistakes that corrupt the result. Operators are human, and shifts are long.
PAS Test vs. Evidentiary Test
It helps to know there are two different breath tests in a DUI stop. The handheld device at the roadside is the preliminary alcohol screening, or PAS, test. For most adult drivers who are not on DUI probation, the PAS test is a voluntary field sobriety test, and you can generally decline it. The evidentiary breath test, usually given at the station on a larger machine, is the one tied to California's implied consent law after arrest, and refusing that one carries real consequences.
People mix these up all the time and think they had no choice at the roadside when they did. The rules around refusal are genuinely confusing, so if that is your situation, read our page on refusing a breath test before you assume anything about your case.
How We Challenge Breath Evidence
Putting it together, here is what a real challenge to a breath reading looks like. We pull the calibration and maintenance logs for the exact device and hunt for gaps and failures. We check whether the 15-minute observation period was truly observed. We look at your medical history for conditions that skew results. We examine the partition ratio assumption. We review the bodycam footage for a burp, a sip, or reflux the officer missed. And where blood was also drawn, we can file a blood split motion to have your sample independently retested by a lab we choose.
Any one of these can put a real dent in the prosecution's case. I want to be clear that none of it is a guarantee, and every case rises or falls on its own facts. But the idea that a breath number ends the discussion is simply not true.
Timing Can Turn a Number Against You
There is one more issue that has nothing to do with the machine breaking, and it trips up a lot of cases: timing. Alcohol takes time to absorb from your stomach into your bloodstream. If you had your last drink shortly before driving, your blood alcohol level may still have been climbing when the officer tested you at the station, which could be an hour or more after the stop.
That gap matters. The law is concerned with your blood alcohol level while you were driving, not at the station. If the number was rising the whole time, your reading at the station can be higher than it actually was behind the wheel. This is the rising blood alcohol defense, and in the right case it can mean you were under the limit when it counted even though the machine later showed you over. It takes evidence and often an expert to develop, but it is a real and recognized argument.
Why You Should Not Just Plead Out
The reason I spend so much time on all of this is simple. People see a number over 0.08 and assume the fight is already lost, so they walk into court ready to plead guilty to whatever the prosecutor offers. That is often the most expensive mistake in the whole process, because a DUI conviction follows you through insurance rates, employment, and any future arrest.
Before anyone pleads to anything, the breath evidence deserves a real look: the machine's records, the observation period, your medical history, the timing of your drinks, and the officer's technique. Sometimes that review supports a full challenge. Sometimes it supports pushing for a reduction to a lesser charge like a wet reckless. Either way, you want to know what the evidence actually holds before you decide, not after. Our overview of the San Diego DUI court process explains where in the timeline these decisions get made.
The Bottom Line
A breath machine produces a number, not a verdict. That number can be wrong because of the machine, the maintenance, the operator, your body, or something as ordinary as a burp at the wrong moment. A high reading is a reason to get good advice, not a reason to give up.
This article is general information, not legal advice, and every case is different. The only way to know whether the breath evidence in your case is solid or shaky is to have someone examine the actual records.
If you were arrested based on a breath test in San Diego and the reading does not sit right with you, call Crudo Law at (858) 622-7280 or reach out through our contact page. The consultation is free and confidential, and Joe Crudo will take an honest look at whether that number holds up.