I am Joel Brand, and I defend DUI cases across California. If you were recently arrested and blew into an evidentiary breath machine at the station, you may have assumed that number was precise and final. In many cases it is neither. One underappreciated issue I examine in almost every breath-test case is radio frequency interference, usually called RFI. This post explains what RFI is, how it can affect the machine that produced your result, and why it matters to your defense.
What Is Radio Frequency Interference?
Radio frequency interference is electromagnetic energy emitted by devices such as police radios, radar units, two-way dispatch systems, nearby cell towers, and even some fluorescent lighting. Breath testing devices contain sensitive electronic components. When electromagnetic energy reaches those components, it can cause the machine to report a BAC reading that is higher than the true alcohol concentration in your breath sample. California law actually recognizes this risk. The Title 17 regulations that govern blood alcohol testing require approved breath instruments to have RFI detectors built in. If the detector trips during your test, the machine is supposed to flag the result as invalid and require a new sample.
Why Police Stations Are Especially Risky Environments
Think about where you took your evidentiary breath test. You were almost certainly inside a police station or a mobile testing unit parked at the curb. Officers carry portable radios that broadcast on the same frequencies known to interfere with breath instruments. Dispatch communications, radar equipment stored nearby, and even the handheld devices officers carry on their belts all create an electromagnetic environment that is far from neutral. The machine sitting on the booking counter may be surrounded by exactly the kind of signals it is designed to detect and reject, but detection is not foolproof.
How the Law Addresses RFI
California's Title 17 regulations require that every approved breath testing instrument be tested for RFI susceptibility before it goes into service. Manufacturers submit data showing that the device can identify interference and abort a corrupted test. The Draeger Alcotest and the Intoxilyzer series used across California are supposed to meet those standards. However, approval during laboratory testing does not guarantee performance in every real-world environment. I routinely request the complete maintenance and calibration packet for the specific machine used in my client's case. That packet often reveals whether the RFI detector has ever been logged as triggering, and whether the device was recertified on schedule.
What the Maintenance Records Can Tell You
Breath machines must be checked and calibrated at regular intervals. The records for those checks are part of the discovery you are entitled to request. I look at maintenance logs for several things related to RFI: whether the detector module was tested separately, whether any anomalous readings were logged around the time of my client's arrest, and whether the machine was due for service but had not yet received it. A gap in calibration history or a skipped RFI verification step is exactly the kind of issue I raise in a motion to challenge the breath evidence. Even if the motion does not result in suppression, it creates leverage during plea negotiations.
The Connection to the Bad Calibration Defense
RFI and calibration problems are closely related. When a machine's electronic components are compromised by outside signals, the calibration standard the machine uses to convert a breath reading into a BAC number may no longer apply accurately. California DUI law recognizes a bad calibration defense specifically because the evidentiary breath test is only as reliable as the hardware and software producing the result. RFI is one mechanism by which calibration can effectively break down in real time, even when the paper record shows the machine passed its last scheduled check.
What About the Roadside PAS Device?
The preliminary alcohol screening device, or PAS, that the officer handed you at the curb is a different instrument from the evidentiary machine at the station. It is also more exposed to RFI because it is used outside, close to the officer's radio and vehicle equipment. California law generally treats the roadside PAS as a screening tool rather than a conclusive result. If you are an adult driver who was not on DUI probation, you had the right to decline the PAS. The result from the station machine is what the prosecution will rely on in court, but understanding how both readings came about matters when I am building your overall defense strategy.
How RFI Arguments Fit Into a Broader Defense
I want to be direct with you. An RFI argument on its own rarely ends a case. It is one piece of a larger picture. When I look at a breath test result, I am simultaneously examining the possibility of mouth alcohol, whether my client's medical conditions could have affected the reading, and whether the three-hour presumption applies properly on the timeline of the stop. RFI becomes most powerful when it combines with other irregularities, such as a result that seems inconsistent with how my client actually performed on field sobriety tests, or a machine that had a maintenance gap in the weeks before the arrest.
What You Should Do Right Now
Time matters in a California DUI case for reasons beyond just RFI. You have a ten-day window from your arrest to request a DMV hearing or you will lose your license automatically without a fight. Separately, the sooner I begin gathering discovery materials, including the breath machine's maintenance logs, the better chance I have of obtaining complete records before anything is lost or overwritten. Do not assume that because a number printed on a receipt that number is accurate. Ask me to look at the machine, its history, and the environment in which your test was taken.
A Note on the Rising BAC Defense and Timing
One reason the precise accuracy of a breath machine matters so much is that prosecutors rely on the test result to establish your BAC at the time you were driving, not just at the time you blew into the machine. If there was any meaningful gap between the stop and the test, the rising BAC defense may already be in play. Add potential RFI distortion to that timeline and the reliability of the number the state wants to use against you becomes genuinely debatable. That kind of debatable evidence is exactly where defenses are built.
What This Means for Your Case Specifically
Every arrest happens in a different location, on a different machine, maintained by a different agency, on a different night. I cannot tell you from this blog post whether RFI affected your result. What I can tell you is that I ask the question in every breath test case I handle. The answer sometimes changes everything, and it sometimes changes nothing. Either way, you deserve to know which applies to your situation before you decide how to proceed. General information like this post is not a guarantee of any outcome in your case.
If you were recently arrested for a DUI in California and you want to understand what the evidence against you actually shows, you can get a free written case analysis on this page. Call me directly at (888) 271-6644. I answer my own phone, 24/7. You can also read more from the DUI blog for additional guidance on where your case goes from here.