California’s breathalyzer law rests on a single trade-off: by driving on a California road, you have already consented to a chemical test of your breath or blood if an officer lawfully arrests you on suspicion of DUI. Refuse that post-arrest test and the DMV suspends your license for a year automatically, whether or not you were actually over the limit.1California Legislative Information. California Vehicle Code 23612 The threshold for being over that limit depends on who you are, and the testing procedures themselves have to follow rules that give you room to challenge a result.
The BAC Limits That Apply to You
California uses three different blood alcohol thresholds depending on the driver:
- 0.08% for drivers 21 and older2California Legislative Information. California Vehicle Code 23152
- 0.04% for commercial drivers and anyone carrying passengers for hire3State of California Department of Motor Vehicles. California Driver’s Handbook – Section 9: Alcohol and Drugs
- 0.01% for drivers under 21 and anyone on DUI probation4California Legislative Information. California Vehicle Code 23136
A reading at or above your applicable limit creates a rebuttable presumption of intoxication when the test is done within three hours of driving.2California Legislative Information. California Vehicle Code 23152 You can challenge it, but you carry the burden of showing why the number is wrong.
How Implied Consent Works After a DUI Arrest
Under Vehicle Code 23612, every driver in California has already agreed to submit to a chemical test of breath or blood if lawfully arrested for DUI.1California Legislative Information. California Vehicle Code 23612 The trigger is a lawful arrest, which requires the officer to have probable cause. Weaving between lanes, an odor of alcohol, and failed field sobriety tests are the kinds of observations that supply it.5Justia. CALCRIM No. 2130 – Refusal – Consciousness of Guilt
After arrest, you generally get to choose between the breath and the blood test. If the officer suspects drugs rather than alcohol, a blood test is required because breathalyzers only detect alcohol.1California Legislative Information. California Vehicle Code 23612 If you’re unconscious or otherwise unable to choose, blood is the default.
You do not have a right to talk to a lawyer before deciding. Formal criminal proceedings haven’t started at the testing stage, so the Sixth Amendment right to counsel hasn’t attached. Trying to delay the test until you can reach an attorney is treated as a refusal.
The Roadside Breathalyzer Is Not the Same Test
The handheld device an officer pulls out at the side of the road is a preliminary alcohol screening (PAS) device. For most drivers 21 and older who aren’t on DUI probation, the PAS is not the evidentiary test covered by implied consent, and you can decline it without triggering refusal penalties.1California Legislative Information. California Vehicle Code 23612 Officers are supposed to advise you of that, though the advisement is easy to miss during a stop.
Two groups don’t get that option. Drivers under 21 and drivers on DUI probation must submit to the PAS test, and refusing it carries the same consequences as refusing a post-arrest chemical test.4California Legislative Information. California Vehicle Code 23136
What Happens if You Refuse the Test
Refusing after a lawful DUI arrest sets off two separate penalty tracks. One runs through the DMV, the other through criminal court. They stack.
Automatic License Suspension
The DMV suspends or revokes your license based on your refusal history over the previous ten years:
- First refusal: one-year suspension6California Legislative Information. California Vehicle Code 13353 (2025)
- Second refusal within ten years: two-year revocation6California Legislative Information. California Vehicle Code 13353 (2025)
- Third or later refusal within ten years: three-year revocation6California Legislative Information. California Vehicle Code 13353 (2025)
This administrative suspension runs independently of the criminal case. If the DA later drops the DUI charge, the refusal suspension stands anyway. Refusal also narrows your options for a restricted license that would otherwise let you drive to work or school, and drivers with a third or later DUI offense in ten years cannot get any restricted license at all.7California Department of Motor Vehicles. Driving Under the Influence (DUI)
Extra Jail Time and Evidence of Guilt
If you refused and are still convicted of DUI, Vehicle Code 23578 lets the court add jail time as a sentencing enhancement: 48 hours for a first offense, 96 hours for a second within ten years, and 10 days for a third. Prosecutors also use the refusal itself as evidence, arguing to the jury that you declined because you knew you were over the limit.5Justia. CALCRIM No. 2130 – Refusal – Consciousness of Guilt
The 10-Day Deadline That Costs People Their Licenses
After a DUI arrest the officer takes your license and hands you a temporary permit. The DMV will then automatically suspend your driving privilege after 30 days unless you request an administrative per se (APS) hearing. You have 10 days from receiving the suspension notice to make that request.8California Legislative Information. California Vehicle Code 13558
Miss the 10 days and the suspension takes effect with no hearing, and there is essentially no way to revive the request afterward. At the hearing itself, you can contest whether the officer had probable cause, whether the arrest was lawful, and whether the testing was done correctly. Winning at the DMV sets aside the license suspension. The criminal case continues on its own track.
Testing Procedures That Can Be Challenged
Title 17 of the California Code of Regulations governs how breath testing devices are maintained and used. Deviations from those rules are where most defenses live.
Calibration Every 10 Days or 150 Tests
Each breathalyzer must be checked for accuracy against a solution of known alcohol concentration every 10 days or every 150 tests, whichever comes first. Operators must also have completed device-specific training.9Cornell Law School. Cal. Code Regs. Tit. 17, 1221.2 – Standards of Procedure When an agency lets a device drift past either threshold without recalibration, every result taken during the lapse is open to challenge.
The 15-Minute Observation Period
Before collecting a breath sample, the officer must observe you continuously for 15 minutes. You cannot eat, drink, smoke, vomit, or burp during that window.10Cornell Law School. Cal. Code Regs. Tit. 17, 1221.1 – Authorized Procedures Any of those can push residual alcohol from the stomach into the mouth and contaminate the reading. Continuous means continuous. If the officer walks off to do paperwork and returns, the observation is broken.
Two Samples Within 0.02
You must provide two separate breath samples, and the two results cannot differ by more than 0.02 grams of alcohol per 210 liters of breath.9Cornell Law School. Cal. Code Regs. Tit. 17, 1221.2 – Standards of Procedure A larger gap suggests something went wrong and may call for additional testing.
Medical Conditions That Skew Breathalyzer Results
A breathalyzer measures alcohol in air exhaled from the lungs, but it can’t distinguish lung air from alcohol vapor that traveled up from the stomach. That’s the mechanism behind most false-high readings.
Gastroesophageal reflux disease (GERD) is the condition most often raised in DUI defense. People with GERD regularly experience stomach contents flowing back into the esophagus and mouth, and any alcohol vapor comes with them. Documented case reports show breath test readings with wavering concentration curves or negative slopes that indicate contamination from stomach alcohol rather than a genuine lung-air reading. Some patients taking proton pump inhibitors or GLP-1 drugs such as semaglutide have reported increased reflux and burping around the time of testing.11NCBI. The Limitations of Mouth Alcohol Detection Systems in Breath Alcohol Testing: Case Reports
The 15-minute observation is meant to catch this, but research shows modern breath analyzers don’t always flag mouth alcohol contamination even when the underlying expirogram data shows it. If you have GERD or a similar condition, contemporaneous documentation of the diagnosis and medications can matter a great deal to any challenge.
Commercial Drivers Face a Separate Track
If you hold a commercial driver’s license, the threshold is half the standard one at 0.04% while operating a commercial vehicle.3State of California Department of Motor Vehicles. California Driver’s Handbook – Section 9: Alcohol and Drugs Refusing a chemical test is itself a disqualifying offense under federal regulations, with or without a DUI conviction.12eCFR. 49 CFR 391.15 – Disqualification of Drivers A first offense brings a one-year disqualification from operating commercial vehicles, a second within three years brings three years, and a drug-related felony involving a commercial vehicle brings lifetime disqualification.13Federal Motor Carrier Safety Administration. Disqualification of Drivers (383.51) For a working driver, a one-year disqualification is usually the end of the job.