If you have ever been pulled over on suspicion of drunk driving in California, you may have been asked to blow into a small handheld device before being arrested. That device is administering a PAS test — Preliminary Alcohol Screening — and understanding what is a PAS test, how it works, and what rights you have regarding it, can make a significant difference in how a DUI investigation unfolds. Many drivers have no idea that the PAS test and the evidentiary breathalyzer test used after arrest are two very different things with very different legal consequences.
This guide breaks down everything you need to know about the PAS test — from how the device functions to how the results are used against you and when they can be challenged in court.
What Is a PAS Test?
A PAS test, or Preliminary Alcohol Screening test, is a roadside breath test administered by law enforcement before a formal DUI arrest is made. It is used as part of the preliminary investigation — alongside field sobriety tests and officer observations — to help determine whether a driver has been consuming alcohol and whether probable cause exists to make a DUI arrest.
The PAS device is a small, portable handheld breathalyzer. The driver breathes into the mouthpiece, and the device measures the concentration of alcohol in the breath, calculating an estimated blood alcohol concentration (BAC). The reading is displayed on the device and recorded by the officer.
Critically, the PAS test is a screening tool — it is not an evidentiary test. Under California law, it is not considered sufficiently reliable on its own to be the primary basis for a DUI conviction. This is why California law requires a separate, more accurate evidentiary chemical test — either a breath test on a larger, more precise breathalyzer at the station or a blood test — after an arrest is made. The distinction between the PAS test and the post-arrest evidentiary test is one that every driver should understand.
How PAS Devices Function
The Screening Process
When a driver blows into a PAS device, the breath sample passes through an electrochemical fuel cell sensor that oxidizes ethanol and measures the resulting electrical current. The strength of the current is proportional to the concentration of alcohol in the breath. The device then calculates an estimated BAC based on the breath-to-blood conversion ratio — a calculation that assumes a standard physiological relationship between breath alcohol concentration and blood alcohol concentration.
This conversion ratio — 2,100:1 — is a population average. It assumes that 2,100 milliliters of alveolar air contain the same amount of alcohol as 1 milliliter of blood. In reality, this ratio varies significantly among individuals and is affected by body temperature, breathing pattern, lung capacity, and other physiological factors. A person whose actual ratio differs from 2,100:1 will receive a breathalyzer reading that does not accurately reflect their true BAC, either higher or lower.
The process typically requires the officer to observe the driver for a minimum of 15 minutes before administering the test to ensure no burping, belching, vomiting, or eating occurs — any of which can introduce mouth alcohol that inflates the reading. If the observation period is not properly maintained, the resulting reading is more susceptible to challenge.
Environmental Factors
PAS devices are designed to detect ethanol, but they can be affected by environmental conditions and substances other than beverage alcohol:
- Temperature: Both the ambient temperature and the temperature of the breath can affect sensor readings. High body temperature from fever or vigorous exercise can elevate breath alcohol readings.
- Chemical exposure: Certain industrial solvents, paints, and cleaning products contain compounds that some breathalyzer sensors may misread as ethanol. Workers with regular occupational exposure to such chemicals may receive falsely elevated readings.
- Mouthwash and breath sprays: Products containing ethyl alcohol can contaminate a breath sample if used shortly before testing, though the required observation period is intended to address this.
- Acid reflux and GERD: Gastroesophageal reflux disease can cause stomach contents — which contain alcohol at concentrations far higher than alveolar air — to reach the mouth and inflate breathalyzer readings significantly.
Understanding PAS Device Use
In California, whether a driver is required to submit to a PAS test depends on their age and the circumstances of the stop:
Adult Drivers (21 and Over)
For drivers 21 and older who have not been arrested, the PAS test is voluntary. California law requires the officer to advise the driver that the test is not mandatory and that they have the right to refuse without losing their driver’s license. This is a critical right that many drivers do not know they have. Declining the PAS test as an adult does not trigger implied consent penalties — those apply only to post-arrest chemical testing.
However, refusing the PAS test does mean the officer has less information about BAC at the roadside, which may cause them to rely more heavily on field sobriety test performance and subjective observations in making the arrest decision.
Underage Drivers (Under 21)
For drivers under 21, the PAS test is mandatory under California’s zero-tolerance laws. Vehicle Code 23136 requires underage drivers to submit to a PAS test when requested by law enforcement during a traffic stop where alcohol use is suspected. Refusing the PAS test as an underage driver triggers the same administrative license suspension as a post-arrest chemical test refusal.
PAS Test vs. Evidential Breathalyzers
Understanding the difference between a PAS test and the evidentiary breathalyzer used after arrest is essential for anyone facing a DUI charge:
PAS Devices
- Small, portable, handheld units
- Used at the roadside before arrest
- Screening tools — results are less reliable and not the primary evidentiary basis for a DUI conviction
- Optional for adults 21 and over; mandatory for drivers under 21
- Results are admissible in court but are typically presented alongside other evidence rather than standing alone
Evidential Breathalyzers
- Larger, stationary units used at police stations or in specially equipped vehicles
- Subject to more rigorous calibration and maintenance standards
- Administered after a lawful DUI arrest under California’s implied consent law
- Results carry greater evidentiary weight and are the primary breath-based evidence in most DUI prosecutions
- Refusal triggers implied consent penalties including license suspension and additional criminal consequences
Role of PAS Results in a DUI Investigation
PAS test results play an important but not always decisive role in the DUI investigation and prosecution process. Officers use the PAS reading — alongside observations of driving behavior, appearance, odor of alcohol, and field sobriety test performance — to establish probable cause for an arrest. If the PAS result is below 0.08%, but the officer believes other evidence indicates impairment, an arrest can still be made.
In court, PAS results are admissible but are typically presented in the context of the full body of evidence. A skilled defense attorney will examine the PAS result critically — looking at the device’s calibration records, the officer’s adherence to required procedures, and whether any alternative explanations for an elevated reading exist.
Scenarios of Heightened Scrutiny
Certain scenarios call for particularly careful scrutiny of PAS test results and the circumstances surrounding them:
BAC Just Above the Legal Limit
When a PAS result is just above 0.08% — say, 0.09% or 0.10% — the margin of error of the device becomes critically important. PAS devices have documented error rates, and a reading that is marginally above the legal threshold may fall within the device’s margin of error, meaning the driver’s true BAC could have been below the legal limit. Challenging the precision of the reading in these cases is a well-established defense strategy.
Time Gap Between Driving and Testing
If significant time elapsed between when the driver was operating the vehicle and when the PAS test was administered — due to a traffic stop on a busy road, a delayed response, or other circumstances — the reading may not accurately reflect the BAC at the actual time of driving. The rising blood alcohol defense may be applicable in these situations.
Inconsistency Between PAS and Evidentiary Test Results
When the PAS result and the post-arrest evidentiary breathalyzer result differ significantly, that inconsistency raises legitimate questions about the reliability of one or both tests. Defense attorneys use such discrepancies to challenge the overall credibility of the breath testing evidence.
Conclusion
The PAS test is a routine part of California DUI investigations, but it is far from the infallible scientific tool it is sometimes portrayed to be. It is a screening device with documented limitations, environmental sensitivities, and procedural requirements that, when not followed correctly, can produce unreliable results. Understanding what is a PAS test — and understanding that you may have rights regarding whether you must submit to one — is knowledge every California driver should have.
If you have been charged with DUI in Bakersfield or Kern County and a PAS test was part of your arrest, contact the Law Office of the Bakersfield Defense Attorney today. We will review every aspect of your case — from the initial traffic stop through the chemical testing procedures — and identify every available defense to fight for the best possible outcome. Do not face a DUI charge without experienced legal representation.
