Defenses- Problems with Roadside Breath Tests or Breathalyzers

The Problems with Breathalyzer Tests

In any DUI investigation, there are typically two types of breath tests that are done. One is commonly referred to as a PAS test (“Preliminary Alcohol Screening” Test), often called a breathalyzer, and the other is referred to as a “chemical” breath test.

This article focuses on problems with roadside, so-called breathalyzer tests (PAS tests).

alcosensor IV

Alcosensor IV

To begin with, you should know that PAS tests/ breathalyzer tests on the one hand and chemical breath tests on the other are very different in terms of   (1) what they prove, (2) how they work and (3) how reliable they are.  The bottom-line is that a PAS test doesn’t prove much, doesn’t work well and is unreliable.  Chemical breath tests are better, but not as accurate as a blood test.

The PAS test is the roadside breathalyzer test.  It is less reliable (see below) and is only supposed to be used to determine if there is alcohol in your blood, not what your BAC is. The chemical breath test is a more sophisticated breath testing device.  It is usually given at the station although a few counties now have chemical roadside breath tests, called “evidentiary PAS machines” meaning that their readings of your BAC can be used as evidence at court, unlike a normal PAS machine.  Evidentiary PAS machines should be avoided whenever possible.

Alcotest 7410 Evidentiary PAS Machine
Alcotest 7410 Evidentiary PAS Machine


The first breath test that you will be offered in what is commonly referred to as a PAS test. California Vehicle Code Section 23612(h) allows for a law enforcement officer to use a “preliminary alcohol screening test” as an additional Field Sobriety Test and investigatory tool to use in a DUI investigation. Before you are offered this test, the officer is supposed to give you a “PAS admonition”. Here is the admonition:

“P.A.S Admonition: I am requesting that you take a preliminary alcohol screening test to further assist me in determining whether you are under the influence of alcohol. You may refuse to take this test; however, this is not an implied consent test and if arrested, you will be required to give a sample of your blood, breath or urine for the purpose of determining the actual alcoholic and drug content of your blood.”

So we learn two things from this admonition: first, you don’t have to take this test—the PAS test is voluntary, unless you are required to take it as a condition of probation from a previous conviction; second, another test—a so-called “chemical” test—is coming.

Problems with the PAS test

Alcosensor V
Alcosensor V

Because you have the option to take the PAS test or refuse it, there are some things that you should know about this device.

Problem 1. The machine isn’t very accurate.

By law, the results of a PAS test cannot be used to establish your BAC. They are designed to detect the presence of alcohol—that is all. Only a chemical test can determine your BAC with any meaningful accuracy.

During 2011, the Santa Clara County District Attorney’s office ordered a review of over 600 DUI arrests occurring between October 2010 and April of 2011 because an AlcoSensor V PAS device operated by the San Jose Police Department was reading falsely and was seriously out of calibration. Since then, all AlcoSensor V machines were taken out of service and replaced by the older, and apparently more reliable AlcoSensor IV. And in a case that I had in Palo Alto Superior Court, the calibration records for an AlcoSensor IV used by the Palo Alto Police Department was out of calibration for nine of twelve months. And it was reading falsely when my client blew into it.

For all of the reasons below, you should never rely on the results of a PAS machine without additional evidence.

Problem 2. A PAS machine runs on a battery.

And batteries run down. And a run down battery will affect the accuracy of the machines report. Very few PAS devices keep a record of what the battery charge is on the machine when you blow into it. One would think that at a minimum, the device would shut down when the battery charge was low, but few do. And very few cops—count them on one hand—write down what the battery charge was in their police report.  I have seen one police report with the battery charge in four years of practicing law.

Problem 3. A PAS machine only tests your breath with a single fuel cell.

A chemical breath test is supposed to test your breath with two separate devices at the same time—an electrochemical fuel cell and an infrared detector. The PAS device only uses an electrochemical fuel cell. The results of the two tests have to be within a range of 0.02 or the test has to be re-done or the machine replaced. This added verification is missing from a PAS device.  If the fuel cell is reading incorrectly, there is no way to know without a separate test or a complete retest of the machine as a whole.

Problem 4. A PAS machine doesn’t have a slope detector, and it needs one to be accurate.

When you blow into a breath machine, first air from your mouth enters the machine, then your throat, esophagus, upper lungs, etc down to the air at the base of your lungs. It is the air above the aveolar sacs at the base of your lungs that gives you the basis for your BAC. The problem is that many machines will include the alcohol that is being blown into the machine from your mouth breath (see GERD and mouth breath alcohol) and that will give you a false high.

Chemical breath machines have a computer program called a “slope detector” which detects the elevated high BAC that the machine reads at the beginning of a blow, and eliminates that false high. PAS machines cannot do this and will report the mouth breath alcohol in your BAC results.

Problem 5. The PAS machine is portable.

It has to be. It is supposed to be used in the field. But this creates a problem. Any PAS machine is going to be subject to the same abuse that any other handheld device receives. PAS machines get dropped, left in hot cars, left in cold cars, coffee gets spilled on them, they get sat on, or their batteries run down, to name just a few. Add to this the fact that a breath machine is a delicate, carefully calibrated and highly sensitive machine, and you have a recipe for disaster.

Problem 6.  The PAS machine shares all of the limitations of all breath machines:

When it comes to an accurate reading of someone’s BAC, there is no substitute for a blood test. Of course, you may be in a situation where you don’t want an accurate reading, but if you are innocent, then a blood test is always the best measure of your BAC. There are a number of reasons for this and they have to do with fundamental problems in the design of all breath machines.  We have enumerated them here.

Legal Limitations to a PAS device

  1. It is easy for an expert to attack
  2. It cannot be introduced into evidence at trial for the purpose of showing what your actual BAC is. It can only be introduced for the purpose of showing that there was alcohol in your blood.

 So why would you want to take a PAS test?

The only reason I can think of, for taking a PAS test is to prove that you have rising blood alcohol. Without a PAS test, you will lose the ability to prove that your blood alcohol was rising and that it was lower than a 0.08 at the actual time of driving. If you drank (from start to finish) during the ½ to one-hour before the stop, then you almost definitely will have rising blood alcohol. A PAS test could prove this.  Without the PAS test, there is no evidence of rising blood alcohol.

What is an evidentiary PAS machine?

Evidentiary PAS machines provide the court with a BAC that can be entered into evidence–unlike a normal PAS machine, which can only prove that there was alcohol in your blood.  These new machines are basically chemical tests that are done in the field.  The problem, again, is the same as with regular PAS devices–portable devices are inherently more unreliable.

The idea behind the evidentiary PAS machine is that it will provide a breath test close to the time of your driving that is more reliable than a PAS test, and more compelling to a jury.  Remember, unlike a PAS test, an evidentiary PAS test can be used to establish your BAC in court.  And because these tests are administered shortly after you were driving, they provide compelling evidence as to what your BAC was at the time you were driving–at least there is a strong bias in that direction.  Currently the Alcotest 7410 is the main evidentiary PAS machine in the field in California, and it is only used in select locations (Central Valley, Santa Cruz County, and a few other counties).

One more time…evidentiary PAS tests should be avoided.  Ask for a blood test instead.  First, the evidentiary PAS device presumes to provide chemical evidence of your BAC from a machine that is portable, battery operated and often out of calibration.  Second, unlike a chemical breath test at the station, the evidentiary PAS device presumes to tell the court what your BAC was close to the time of driving, eliminating any rising blood alcohol defense.  Third, these machines have not had enough time in the field to provide any evidence as to their actual reliability in the field.  It always happens that new breath machines are first hailed as infallible, then brought gradually under more scrutiny until the truth comes out.  At present, all we are left with is the manufacturer’s manual (and limited warranty!).  But all of the problems with breath machines remain.  These machines are hardly something that you can rely on with confidence.