DUI cases are won and lost on evidentiary challenges. The prosecution’s case — driving observations, field sobriety test performance, breath or blood test results, and the arresting officer’s testimony — looks formidable on paper, but each component is built on a foundation of constitutional requirements, regulatory rules, and scientific assumptions that can be tested. California Code of Regulations Title 17 governs the admissibility of chemical tests with specific mandatory procedures. The Fourth Amendment governs the legality of the stop and any searches. NHTSA standards govern how field sobriety tests must be administered. When any of these foundations is weak, evidence can be excluded — and an excluded breath test or an excluded statement often ends the case. The Law Office of Nic Cocis defends DUI cases throughout Murrieta, Temecula, Menifee, Lake Elsinore, Wildomar, Winchester, Canyon Lake, and French Valley, and applies layered evidentiary challenges in every case.
Challenging the Traffic Stop — Fourth Amendment Foundations
IEvery DUI case begins with a stop. The constitutional requirement is reasonable suspicion that a traffic violation has occurred or that criminal activity is afoot. If the stop was not supported by reasonable suspicion, everything that flowed from it — the officer’s observations, the field sobriety tests, the breath test, the statements made — is subject to suppression as fruit of the unlawful stop.
A motion to suppress under California Penal Code § 1538.5 is filed before trial and asks the court to exclude evidence obtained as a result of an illegal stop or search. Common stop challenges in Riverside County DUI cases:
- Pretextual or invented violations. The officer claims a vehicle code violation that the body or dash camera footage does not actually support
- Weaving within a lane. California courts have held that weaving within a single lane, without crossing the line, is not by itself reasonable suspicion for a DUI stop
- Anonymous tip stops without corroboration. Anonymous tips must contain sufficient indicia of reliability and be corroborated before forming the basis of a stop
- Sobriety checkpoint violations. California sobriety checkpoints must meet the eight-factor test set out in Ingersoll v. Palmer (1987) 43 Cal.3d 1321 — supervisory decision-making, neutral pattern of stops, advance notice, and others. Checkpoints that fail to meet these requirements produce suppressible evidence
A successful § 1538.5 motion typically results in dismissal of the case — there is rarely enough remaining evidence after suppression to support a DUI charge.
Challenging the Field Sobriety Tests
Field sobriety tests (FSTs) are voluntary, and most are not scientifically validated as DUI indicators. The National Highway Traffic Safety Administration (NHTSA) has validated only three standardized FSTs: the Horizontal Gaze Nystagmus (HGN), the Walk-and-Turn, and the One-Leg Stand. Non-standardized FSTs — the alphabet recitation, counting backward, finger-to-nose — have no scientific support.
The Horizontal Gaze Nystagmus test. HGN is the most “scientific-looking” of the FSTs but has substantial reliability problems. Nystagmus — involuntary jerking of the eyes — can be caused by:
- Medical conditions (inner ear disorders, brain injury, certain neurological conditions)
- Medications and prescription drugs
- Eye trauma or eye fatigue
- Caffeine or nicotine
- Genetic baseline nystagmus unrelated to impairment
The test must be administered with the eyes at a specific angle, at a specific distance, with specific stimulus movement. Officers who deviate from the NHTSA protocol produce unreliable results. Many California officers are not currently NHTSA-certified for HGN administration.
Walk-and-Turn and One-Leg Stand. These tests measure balance and divided attention. Performance is affected by:
- Medical conditions (back, knee, hip, inner ear issues, neuropathy)
- Footwear (heels, boots, flip-flops)
- Surface conditions (slope, uneven pavement, gravel)
- Age (NHTSA studies excluded participants over 65)
- Weight (NHTSA studies excluded participants 50+ pounds overweight)
- Time of day and fatigue
A driver who has any of these factors can fail FSTs while completely sober. The defense work in challenging FSTs typically includes obtaining the officer’s training and certification records and reviewing body camera footage frame-by-frame to confirm proper administration.
Challenging the Breath Test — Title 17 Compliance
Breath alcohol testing in California is governed by Title 17 of the California Code of Regulations. Title 17 sets specific mandatory procedures, and failure to follow them can result in the breath test being excluded from evidence.
The 15-minute continuous observation period. Under 17 CCR § 1219.3, the officer must continuously observe the driver for a minimum of 15 minutes immediately before the breath test, with no eating, drinking, smoking, vomiting, or regurgitation. The purpose is to eliminate mouth alcohol — alcohol from residual drinks, food, breath spray, dentures, GERD reflux, or other sources — that would contaminate the reading. Officers regularly fail to actually observe the driver for the full 15 minutes (they leave the room, do paperwork, or process other arrestees). When the defense establishes that continuous observation did not occur, the breath test can be excluded.
Mouth alcohol contamination. Even with a 15-minute observation period, certain conditions can introduce mouth alcohol that the officer cannot detect:
- Gastroesophageal reflux disease (GERD). Reflux during the test can introduce alcohol from the stomach into the breath sample
- Dentures and dental work. Bridges, crowns, and dentures can trap residual alcohol from a recent drink
- Recent oral substances. Mouthwash, breath spray, cough drops with alcohol bases
- Asthma inhalers. Some inhalers contain alcohol
Calibration and maintenance records. Title 17 requires breath testing devices to be calibrated at specific intervals — generally every 10 days or 150 tests, whichever comes first. Records of calibration, accuracy verification, and maintenance must be available. The defense can subpoena these records, and devices outside calibration tolerance produce excludable results.
The 2100:1 partition ratio assumption. California’s breath test relies on the statutory assumption that the ratio of alcohol in deep lung breath to alcohol in blood is 2100:1. In reality, the actual partition ratio varies among individuals — studies have found ratios ranging from approximately 1300:1 to 3100:1. A defendant with a low partition ratio will have a breath result that overstates the actual blood alcohol concentration.
Source code and machine challenges. California uses several breath testing devices (the CMI Intoxilyzer and others). Source code review challenges have been raised, with mixed results in California courts. Where a specific device has documented reliability problems, the defense can move to exclude its results.
For a deeper treatment of breath and blood test evidence challenges generally, see our breath and blood tests practice area page.
Challenging the Blood Test
Blood tests appear more reliable than breath tests but have their own vulnerability points:
Phlebotomist certification. Under 17 CCR § 1219.3, the blood draw must be performed by a certified person — typically a phlebotomist, registered nurse, licensed vocational nurse, or physician. Where the person who drew blood was not properly certified at the time of the draw, the test result can be excluded.
Chain of custody. Blood samples pass through multiple hands — the phlebotomist, the booking officer, the evidence technician, the lab analyst. Each transfer must be documented. Gaps in the chain create reasonable doubt about whether the sample tested was the sample drawn from the defendant.
Fermentation in the vial. Blood samples must be drawn into vials containing both an anticoagulant (typically potassium oxalate) and an antimicrobial preservative (typically sodium fluoride). If either is missing or insufficient, fermentation can occur in the vial — producing alcohol that was not actually in the defendant’s blood at the time of the draw. False high readings of 0.02-0.04% can result.
Skin contamination. Standard practice requires cleaning the draw site with a non-alcohol antiseptic. Where alcohol-based swabs are used, the alcohol can contaminate the blood sample during draw.
Hospital blood vs. forensic blood. Blood drawn in a hospital for medical purposes — particularly using serum or plasma rather than whole blood — is not directly comparable to forensic whole-blood testing. Serum results overstate whole-blood BAC by approximately 14%, a critical difference where the result is near the legal threshold.
Independent retesting. California Vehicle Code § 23158 allows the defense to obtain an independent test of the blood sample. Where the independent test produces a different result, the defense has direct evidence to challenge the prosecution’s number.
The Rising BAC Defense
This is one of the most important DUI defenses available and one that the original draft of this post missed entirely.
The crime under Vehicle Code § 23152(b) is driving with a BAC of 0.08% or higher. The relevant time is the time of driving, not the time of testing. But the breath or blood test is typically administered 30 minutes to two hours after the stop — well after the driving has ended.
Alcohol absorption is not instantaneous. After a drink is consumed, alcohol absorbs from the stomach and intestines into the bloodstream over a period of 30 minutes to two hours. A driver who consumed alcohol shortly before driving may have a substantially lower BAC at the time of driving than at the time of testing — because absorption is still occurring while the driver is on the road.
The defense theory: the test result of 0.10% taken 90 minutes after the stop does not prove the driver was at 0.10% while driving. With expert testimony from a forensic toxicologist, the defense can model the absorption curve and argue that the driver was likely below 0.08% at the time of driving.
This defense is particularly effective when:
- The defendant had recently finished drinking before the stop
- A significant time gap elapsed between the stop and the test
- The defendant has any reason to attribute the post-driving BAC to continued absorption rather than steady state
- Expert testimony can support a specific absorption curve
The rising BAC defense does not work in every case, but it is a powerful tool in cases with the right facts.
Challenging the Officer’s Testimony and Personnel File
The arresting officer’s testimony — about driving observations, the smell of alcohol, slurred speech, red eyes, FST performance — is subjective and contestable. Two specific tools the defense uses:
Pitchess motions (Evidence Code § 1043). A Pitchess motion seeks the personnel records of the arresting officer to identify prior complaints of dishonesty, false reporting, excessive force, or other relevant misconduct. Where granted, the records can be used to impeach the officer at trial. The Riverside County Sheriff’s Department, Murrieta Police Department, and Temecula Police Department all maintain personnel records subject to Pitchess discovery.
Body camera and dash camera review. Every modern DUI arrest is recorded. The defense should request all video evidence in discovery and compare it against the officer’s narrative. Inconsistencies — driving that looks normal on video but is described as erratic, FST performance that looks reasonable but is described as failed, statements not actually made — provide concrete impeachment material.
Constitutional and Procedural Challenges
Beyond the foundational stop and test challenges, additional constitutional and procedural issues frequently arise:
Miranda violations. Statements made during custodial interrogation without proper Miranda warnings are suppressible. The Miranda analysis turns on whether the defendant was in custody — a fact-specific question that depends on the circumstances of the interaction.
Implied consent admonition. Under VC § 23612, officers must properly advise the driver of the consequences of refusing a chemical test. Where the admonition was incomplete or improperly delivered, a claimed refusal may not be valid. We discuss the implied consent framework in detail in our California implied consent post.
Trombetta motions. Under California v. Trombetta (1984) 467 U.S. 479, the prosecution has a duty to preserve materially exculpatory evidence. Where evidence has been lost or destroyed — body camera footage that was deleted, breath test machine logs that weren’t preserved, a blood sample that was disposed of before independent testing — sanctions can include exclusion of related evidence or dismissal.
Confrontation Clause and lab analysts. Under Melendez-Diaz v. Massachusetts (2009) 557 U.S. 305 and Bullcoming v. New Mexico (2011) 564 U.S. 647, the prosecution must produce the actual lab analyst who performed the test for cross-examination. Surrogate testimony from a different analyst is generally not sufficient. Where the prosecution cannot produce the actual analyst, lab results can be excluded.
Talk With a Riverside County DUI Defense Attorney
DUI defense in 2026 is technical work. The challenges available — Fourth Amendment, Title 17, NHTSA standards, partition ratio, rising BAC, Pitchess, Trombetta, Confrontation Clause — require systematic review of every stage of the case. A defense that does not layer these challenges leaves substantial exposure unaddressed.
For comprehensive treatment of DUI evidence challenges, see our challenging DUI evidence practice area page and our DUI defense practice area.
Learn more about attorney Nic Cocis and his 25+ years of trial experience defending Riverside County DUI cases.
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