Challenging Breathalyzer Results: RFI Evidence and Suppression

Challenging breathalyzer results with an RFI defense means attacking the reading itself by showing that radio frequency interference at the testing scene likely pushed the number higher than your actual blood alcohol concentration, and that the machine’s own interference detector was not built to catch the signals present. The defense has real traction because the RFI detection circuitry in the most widely deployed breathalyzers was engineered for frequency ranges that modern police radios and cell phones no longer use.¹ Done right, the challenge ends with a pretrial motion to suppress that strips the prosecution of its strongest piece of evidence.

Why the Detection Gap Matters

Breathalyzer manufacturers install RFI detection circuits that are supposed to sense radio energy in the environment and abort the test when interference appears.² The problem is what those detectors watch for. The RFI detection circuitry in the Intoxilyzer series, including the widely deployed 5000, 8000, and 9000 models, was designed to sense radio frequencies in the 10 to 300 MHz range.¹

Modern equipment operates well above that. Police radios today transmit between 400 MHz and 3 GHz, cell phones use 800 MHz through 1.9 GHz, and Bluetooth devices sit around 2.4 GHz.¹ The machine’s built-in safeguard is essentially blind to the signals most likely to be around it during a breath test.

The physical range of the detector compounds the problem. Testing has shown an interference source generally has to be within about two inches of the external antenna to trigger the internal warning. A police radio keyed three feet away, close enough to inject stray voltage into the machine’s circuitry, produces no error message at all. In one controlled experiment, the Intoxilyzer 9000 identified active cell phone calls only two out of five times, even at close range.¹

The bump this can produce is not theoretical. During testing on a subject with zero alcohol, one widely used model produced back-to-back readings of 0.000 and 0.026 g/dL while a cell phone call was active nearby, without flagging any RFI error.¹ For a driver who actually blew a legal 0.06, that same interference could push the reported reading over the 0.08 threshold.

How Interference Corrupts the Reading

Breathalyzers use either infrared spectroscopy or a fuel cell to measure alcohol, and both translate a physical measurement into an electrical signal that a microprocessor converts to a displayed number. Electromagnetic waves from a nearby radio or phone penetrate the housing and induce voltage in the internal wiring. The microprocessor cannot tell the difference between that stray current and the legitimate signal from the sensor. An infrared machine registers more light absorption than actually occurred. A fuel cell machine reads a higher electrical charge than the chemical reaction produced. The number on the printout goes up either way.

Sources of Interference at the Scene

A typical traffic stop is saturated with radio signals. Police cruisers carry mobile digital terminals, in-car radios, and automated license plate readers that all transmit data. The officer standing next to the machine usually has a portable radio on their vest and a phone in their pocket. Personal devices in the vehicle, including phones and Bluetooth accessories, emit bursts of radio energy whether they’re being actively used or not.

Station testing has its own environment. Commercial broadcast infrastructure, microwave equipment in break rooms, and building security systems can generate localized electromagnetic fields. The risk climbs sharply when any transmitting device sits within a few feet of the breathalyzer during sampling.

The observation period matters here too. Most states require the officer to observe you continuously for 15 to 20 minutes before the test, and during that window officers routinely talk to dispatch by radio and run your license through the mobile terminal. Every one of those transmissions puts radio energy into the space around the machine and, if dispatch logs show it, can be documented later.

The Records You Need Through Discovery

A workable challenge depends on reconstructing the electromagnetic environment at the moment of the test. Most of that evidence sits with law enforcement and has to be pulled through discovery.

Breathalyzer Maintenance and Calibration Logs

A subpoena duces tecum compels the law enforcement agency or state forensic lab to produce the machine’s service history.³ These records show whether the device has a history of RFI errors, sensor malfunctions, or repairs to its shielding. Technician notes from certification inspections sometimes document shielding issues that were never addressed.

Radio Transmission and Dispatch Logs

Police Computer Aided Dispatch systems keep timestamped logs of all radio communications and synchronize their timestamps against a master clock for accuracy across workstations.⁴ Comparing the printout timestamp on the breath test against dispatch records and the officer’s own radio activity can show whether a transmission occurred during or immediately before the sampling window. A radio keyed within seconds of the sample is strong evidence of interference.

Event Logs From the Breathalyzer

Modern breathalyzers store internal event logs recording detector activations, test aborts, and error codes. Even where the detector missed interference from modern frequencies, partial activations or intermittent flags on nearby tests by the same machine can demonstrate instability. Logs showing “Radio Interference” or “RFI” errors on other tests build the case that the device operates in an environment where interference is a recurring problem.²

Body Camera and Dashcam Footage

Video can establish where the officer’s radio and phone were physically located relative to the breathalyzer during the test. If footage shows the officer keying a radio next to the machine, or a phone sitting on the same surface, that visual anchors the technical argument. Photos of the testing location covering nearby electronic equipment and broadcast infrastructure fill in the rest.

Filing the Motion to Suppress

The formal challenge takes the shape of a pretrial motion to suppress, asking the judge to exclude the breathalyzer result. Under federal rules and most state equivalents, suppression motions must be filed before trial begins, with a deadline typically set at or shortly after arraignment. If no specific deadline is set, the cutoff is the start of trial itself.⁵

A late motion is treated as untimely, and the court will only consider it if you can show good cause for the delay.⁵ Waiting on discovery or expert analysis is not automatically good cause, so the motion should be filed as early as the evidence allows. Filing early also forces the prosecution to answer with its own evidence of reliability, which often exposes further weaknesses.

The motion argues that the test was conducted in an uncontrolled electromagnetic environment, that the device’s RFI detector was incapable of identifying the frequencies present, and that the resulting measurement is scientifically unreliable. The prosecution then bears the burden of showing the result was obtained under conditions that ensure accuracy.

The Admissibility Standard

Before a breathalyzer result reaches a jury, the judge decides whether the evidence is reliable enough to be admitted. A majority of states and all federal courts apply the Daubert standard, under which the judge weighs whether the technique has been tested, subjected to peer review, has a known error rate, operates under maintained standards, and has gained acceptance in the relevant scientific community.⁶ An RFI challenge under Daubert focuses on the error rate and on the failure of the instrument’s operating standards to account for modern frequency ranges.

A smaller group of states still uses the older Frye standard, which asks whether the underlying scientific principle is generally accepted by experts in the field. Under Frye, the argument is that the breathalyzer’s RFI detection system does not reflect generally accepted science because it ignores the frequencies that dominate the current electromagnetic environment. Other states apply hybrid tests. All of them demand some showing of reliability before the number comes in.

The Evidentiary Hearing

At the pretrial hearing, both sides present evidence on reliability. The defense puts on the records collected through discovery: radio logs showing transmissions during the test, the machine’s maintenance history documenting prior RFI events, and internal event logs. An expert then ties it together, explaining how the frequencies documented in the officer’s transmissions fall outside the detection range of the model used, and walking the judge through the published testing showing a false 0.026 reading on a sober subject during active cell phone interference.¹

The prosecution usually responds with testimony from the officer about where devices were located and from the agency’s breath test technician about calibration. Proximity and timing of transmissions relative to the sample are the central factual questions the judge has to resolve.

The Expert You Need

An RFI defense almost always requires an expert witness. The expert needs qualifications in electrical engineering, forensic toxicology, or a related field, and must show specific familiarity with the breathalyzer model at issue. Under the federal rule and its state analogs, an expert qualifies through knowledge, skill, experience, training, or education, and the testimony must rest on sufficient facts, use reliable methods, and apply those methods reliably to the specific case.⁷

Hourly rates for forensic experts in DUI cases generally run $250 to $350 per hour for consultation and testimony, with court appearances at the higher end. Most require an upfront retainer, with a typical median around $2,000 before any work begins. A full engagement covering file review, report preparation, and hearing testimony can run several thousand dollars. For a first-offense DUI where the reading was close to 0.08, the expert is often the single largest defense expense and also the piece most likely to change the outcome.

Under the 2023 amendment to the federal expert testimony rule, forensic experts should avoid claiming absolute or 100% certainty when the methodology involves any subjective element.⁷ A good expert frames the testimony in terms of probability and documented error rates rather than sweeping guarantees about what the interference did.

What Suppression Actually Changes

If the motion succeeds, the prosecution loses its most quantifiable evidence. The case shifts to field sobriety test performance, the officer’s observations about your appearance and behavior, and any statements you made during the stop. None of those carry the same weight with a jury as a machine-generated number above 0.08.

The prosecution can still pursue the case on that softer evidence, particularly if the officer documented clear signs of impairment. If a blood draw was taken alongside the breath test, those results remain available unless independently challenged. In some jurisdictions, prosecutors facing a suppressed breath test will offer a plea to a reduced charge rather than try the case on observation evidence alone.

Suppression is not the same as dismissal. But a case built on “the officer thought you seemed impaired” is a different case than one built on “the machine says 0.11,” and defense attorneys who do the technical work to remove the number from the file often find plea discussions look very different afterward.

1
Counterpoint Journal. Performance Characteristics of the Intoxilyzer 9000
2
ResearchGate. Radio Frequency Interference Messages During Breath Testing of Suspected Impaired Drivers Using the Intoxilyzer 5000C
3
Virginia Department of Forensic Science. Subpoenas Duces Tecum (SDTs)
4
Bureau of Justice Assistance. Standard Functional Specifications for Law Enforcement Computer Aided Dispatch Systems
5
Legal Information Institute. Federal Rules of Criminal Procedure Rule 12 – Pleadings and Pretrial Motions
6
Legal Information Institute. Daubert Standard
7
Legal Information Institute. Federal Rules of Evidence Rule 702 – Testimony by Expert Witnesses