A breathalyzer is a machine that estimates blood alcohol from your breath — and machines make mistakes.
Several well-known error sources can make a breath result unreliable, and each is a place to fight a DUI.
Mouth Alcohol
A breath test is supposed to measure deep-lung air. If alcohol lingers in the mouth — from a recent drink, a burp, acid reflux (GERD), dentures, or mouthwash — it can spike the reading far above your true blood level. This is why officers are supposed to observe you for about 15 minutes first; if they didn’t, the result is suspect.
Calibration and Interference
Breath machines must be regularly calibrated and maintained, with logs to prove it. A missed calibration, a maintenance gap, or records the prosecution can’t produce undermine the result. Radio-frequency interference and a machine out of tolerance can also skew readings.
The Partition-Ratio Assumption
Breathalyzers convert breath alcohol to blood alcohol using a fixed ratio (2,100:1) that assumes an average person. Real people vary — by body temperature, breathing pattern, and physiology — and for many, that assumption overstates the true blood level.
This challenge is available only on the impairment count, VC 23152(a). Under People v. McNeal (2009) 46 Cal.4th 1183, the ratio is built into the definition of the VC 23152(b) “per se” 0.08% breath offense, so partition-ratio evidence is inadmissible to attack the (b) count. Where a case rests on a single borderline breath reading, this and other errors can mean a dismissal or a wet reckless.
Frequently Asked Questions
Can a breathalyzer be wrong?
Yes. A breathalyzer is an estimate, not an exact reading, and several known error sources can throw it off: mouth alcohol, missed calibration or maintenance, radio interference, and the built-in partition-ratio assumption. That last one only helps on the VC 23152(a) impairment count — under People v. McNeal (2009) 46 Cal.4th 1183, it cannot be used against the 23152(b) “per se” charge.
What is mouth alcohol in a DUI case?
If you burped, took a recent sip, have GERD, wear dentures, or used mouthwash before you blew, your reading may not reflect your actual blood alcohol — mouth alcohol can spike it artificially high. The fix is procedural: officers must watch you for about 15 minutes before testing to rule this out, and if that observation period was skipped or rushed, the result is worth challenging.
How do you challenge a breath test in a DUI?
Every breath number has four soft spots worth checking: contamination from mouth alcohol, gaps in calibration and maintenance records, radio interference, and — for the VC 23152(a) impairment count only — the partition-ratio assumption, which is barred against the 23152(b) “per se” count under People v. McNeal (2009) 46 Cal.4th 1183. On a close case, any one of these can move the needle toward a reduction or dismissal.
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Ahmed S. Hasan
San Francisco Criminal Defense Attorney · State Bar of California #364992
Ahmed S. Hasan is the founder of ASH Legal, a solo criminal defense practice based in San Francisco. The firm runs on flat-fee representation: one lawyer, one fee, from arraignment through resolution. No hourly billing. No surprise invoices.
Ahmed is a graduate of Emory University School of Law and a member in good standing of the State Bar of California (Bar #364992). His practice focuses on San Francisco misdemeanor defense — DUI, domestic violence, petty theft and shoplifting, drug possession, vandalism, trespassing, and assault and battery — with particular attention to diversion-track outcomes that end in dismissal under PC 1001.95, PC 1001.36 (Mental Health Diversion), PC 1001.80 (Military Diversion), and PC 1001.83 (Parental Caregiver Diversion).
Ahmed previously served as a post-bar clerk with the San Francisco Public Defender's Office, the city's largest indigent-defense practice. He is a member of the Asian Pacific American Bar Association of Silicon Valley.
He represents clients at the San Francisco Hall of Justice (850 Bryant Street) and the Civic Center Courthouse. The ASH Legal office is at 15 Boardman Place, Suite 301, San Francisco, CA 94103.
Free 30-minute consultations are available by phone or Zoom. (510) 545-6515 · ahmed@ashlegal.com