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Challenging Breathalyzer Accuracy in Chicago DUI Cases: What Defendants Need to Know
Why Breathalyzer Evidence Is Central in Chicago DUI Cases
In Chicago, DUI arrests are common along the Eisenhower Expressway, Lakeshore Drive, and in neighborhoods like Pilsen, Uptown, and Austin. Illinois law enforcement agencies rely heavily on breathalyzer machines to secure DUI convictions. Under 625 ILCS 5/11-501, a driver with a blood alcohol concentration (BAC) of 0.08% or higher is presumed legally intoxicated. For drivers under 21, Illinois enforces zero tolerance, meaning any trace of alcohol can lead to charges.
But unlike blood tests, which directly measure alcohol content, breathalyzers only estimate BAC by analyzing a breath sample. This means their results are not always accurate. Because Illinois courts allow breathalyzer evidence, prosecutors treat it as one of the strongest tools for conviction. That makes challenging the accuracy of these results one of the most important parts of a defense strategy.
How DUI Cases Begin in Illinois and the Role of the Breath Test
Most Illinois DUI cases begin with a traffic stop. Officers may pull a driver over for weaving, speeding, or another traffic violation. In Chicago, officers often document every step on dashcams or body-worn cameras. During the stop, they look for signs of impairment such as slurred speech, bloodshot eyes, or the smell of alcohol.
If impairment is suspected, field sobriety tests may follow. If those are “failed,” the officer may request a preliminary breath test (PBT). PBT results cannot be used in court to prove intoxication but can provide probable cause for an arrest.
Once at the police station, the driver is offered an evidentiary breath test on a certified machine such as the Intoximeter EC/IR II. This test, unlike the PBT, is admissible in court. But Illinois’ implied consent law (625 ILCS 5/11-501.1) means that refusal triggers a statutory license suspension, often longer than if the test is taken and failed.
The results from that machine often form the centerpiece of the prosecution’s case. But the science behind breathalyzers is less precise than most jurors believe.
Why Breathalyzer Results Can Be Wrong
Breathalyzers calculate BAC using a formula that assumes a uniform ratio between alcohol in deep lung air and alcohol in the blood. In reality, that ratio can vary widely between individuals. Factors that can lead to false positives include:
- Improper calibration: Illinois regulations require breathalyzer machines to be certified every 62 days. A lapse in certification can invalidate results.
- Operator error: Officers must follow a 20-minute observation period before testing to prevent mouth alcohol contamination. Skipping steps can affect accuracy.
- Medical conditions: Acid reflux, diabetes, and even asthma inhalers can cause false readings.
- Environmental factors: Residual mouth alcohol from mouthwash or dental work, as well as radio frequency interference, can alter results.
- Dietary influences: Low-carb or ketogenic diets produce acetone, which can be misread as ethanol by some machines.
Because of these variables, Illinois law allows defendants to challenge breathalyzer results in court.
Penalties for DUI in Illinois
The penalties for DUI in Chicago depend on whether the case is charged as a misdemeanor or felony. A first offense is typically a Class A misdemeanor, carrying up to one year in jail, fines of up to $2,500, and driver’s license suspension. A second offense can involve mandatory minimum jail or community service.
An Aggravated DUI — such as a third conviction, DUI with a minor passenger, or DUI resulting in great bodily harm — is charged as a felony. Felonies carry prison terms ranging from 1 to 7 years or longer, along with fines and permanent license revocation.
Collateral consequences are equally damaging: increased insurance rates, permanent criminal record, difficulty finding employment, and potential loss of professional licenses.
A Chicago Example: How Defense Can Win
Consider a driver stopped in Bronzeville for allegedly failing to signal. The officer claims the driver smelled of alcohol and had slurred speech. After an arrest, the station breathalyzer recorded a BAC of 0.10.
The defense attorney subpoenaed calibration records for the machine and discovered it had not been tested within the 62-day period required by Illinois Administrative Code. Additionally, bodycam footage showed the officer failed to observe the driver continuously for the required 20 minutes before the test.
Through motions to suppress, the breathalyzer results were excluded. With no scientific evidence, the State relied only on the officer’s observations, which the jury found insufficient to prove impairment beyond a reasonable doubt. The driver was acquitted.
The Trial Process in Illinois DUI Cases
DUI trials in Cook County often hinge on whether the breath test results are admitted. The process includes:
- Arraignment: Defendant is formally charged.
- Pretrial motions: Defense may move to suppress breathalyzer evidence.
- Discovery: Both sides exchange police reports, videos, and machine maintenance logs.
- Trial: Prosecutors present officers and technicians; defense cross-examines and presents expert witnesses.
- Verdict and sentencing: Judge or jury decides guilt; penalties are imposed if convicted.
A skilled attorney scrutinizes every aspect of the test and the State’s case to create reasonable doubt.
Defenses Against Breathalyzer Evidence
Defenses vary but commonly include:
- Proving the machine was not properly calibrated or certified
- Demonstrating officer error during testing procedure
- Presenting medical evidence explaining false positives
- Arguing chain-of-custody issues for records and test results
- Showing video evidence inconsistent with claimed impairment
These defenses require technical knowledge and often expert testimony.
FAQs About Challenging Breathalyzers in Chicago DUI Cases
Can I win a DUI case if I blew over 0.08?
Yes. Breathalyzer results can be excluded or discredited if procedures were not followed or if scientific weaknesses are exposed.
What happens if I refuse the breath test?
Your license will be suspended automatically — 12 months for a first refusal, 36 months for a subsequent refusal. But refusal may limit the State’s evidence at trial.
Are portable breath tests the same as evidentiary tests?
No. Portable tests are less reliable and not admissible in court to prove intoxication. Only evidentiary machines used at the station are.
How often are breathalyzers wrong?
Studies suggest that when machines are not maintained or procedures are skipped, error rates can be significant. Even small errors matter when the legal limit is 0.08.
Can medical conditions really affect results?
Yes. Acid reflux, diabetes, and diets can produce compounds that mimic ethanol. This is a recognized defense in Illinois courts.
If I’m convicted, can I ever clear a DUI from my record?
No. DUI convictions in Illinois are permanent and cannot be sealed or expunged.
Why Legal Representation Is Critical
Without an attorney, defendants face prosecutors who will argue the breathalyzer is unquestionable science. A skilled attorney knows how to demand maintenance logs, review officer certifications, retain independent toxicologists, and present these flaws to a judge or jury.
The Law Offices of David L. Freidberg has decades of experience defending clients in Chicago and surrounding counties against DUI charges, including cases where breathalyzer evidence was the centerpiece.
Why Choose The Law Offices of David L. Freidberg
Defendants in Chicago and throughout Illinois face harsh penalties if convicted of DUI. Choosing the right attorney is critical. The Law Offices of David L. Freidberg has decades of experience, is available 24/7, and has successfully defended countless DUI cases across Cook, DuPage, Will, and Lake Counties.
When You Need a Fighter, Call Us!
If you were arrested in Chicago, protect your future by contacting The Law Offices of David L. Freidberg. We have decades of experience handling DUI cases in Illinois. Our firm is available 24/7 to provide the legal defense you deserve.
Contact us today at (312) 560-7100 or toll-free at (800) 803-1442 for a free consultation. We’re available 24/7 to serve clients throughout Illinois, Cook County, DuPage County, Will County, and Lake County.