How to Challenge a DUI: Field Sobriety, Breathalyzer, and Checkpoint Defenses

A DUI charge is not the same as a DUI conviction. Prosecutors have to prove every link in the chain: that the stop was lawful, that the testing was done correctly, and that the numbers actually mean what they claim. Each of those links can break. Below is a plain-English map of where most DUI cases are won or lost, so you know what a defense looks like and when to bring in a lawyer.

Start with the stop itself

Before anything an officer observed about your driving matters, the stop has to be legal. Under the Fourth Amendment, police need reasonable suspicion of a traffic or criminal violation to pull you over. The Supreme Court in Delaware v. Prouse struck down random, suspicionless stops of individual drivers. If the officer had no lawful reason to stop you in the first place, everything that follows, the field tests, the breath result, even your statements, can be thrown out under the exclusionary rule.

This is why dashcam and bodycam footage matters so much. A report that says you were "weaving" looks very different when the video shows you holding your lane. Drifting once within your lane, briefly touching a line, or slowing near a parked patrol car often is not enough. A good defense starts by testing whether the articulated reason for the stop actually holds up.

Challenging field sobriety tests

The three Standardized Field Sobriety Tests (SFSTs) come from a federal training program: the horizontal gaze nystagmus (eye) test, the walk-and-turn, and the one-leg stand. They are validated only when administered exactly the way officers are trained, and only the standardized clues count. That precision is exactly where a field sobriety test defense lives.

• Administration errors. Giving instructions wrong, scoring non-standard "clues," or running the test on a slope, gravel, or in heels undercuts the result. Bodycam footage is often compared frame by frame against the training manual.
• Medical and physical conditions. Inner-ear problems, old injuries, back or knee issues, obesity, age over 65, and neurological conditions all affect balance tests regardless of alcohol.
• The eye test. Nystagmus has many innocent causes and is hard for an officer to read accurately on the roadside. Many courts limit how it can be described to a jury.
• They are voluntary. In most states you are not legally required to perform roadside SFSTs, and refusing them usually carries no automatic license penalty (unlike refusing the post-arrest chemical test). Politely declining gives the officer less to build on.

Attacking the breathalyzer

Breath machines are scientific instruments, and instruments fail. A breathalyzer defense usually targets the device, the operator, or the biology.

• Calibration and maintenance records. Most states require breath machines to be calibrated and certified on a schedule. Missing logs, overdue calibration, or a known-faulty unit can make the reading inadmissible. Your lawyer subpoenas these records.
• The 15-minute observation period. Officers are supposed to watch you before the test to be sure you did not burp, vomit, or regurgitate, which can push mouth alcohol into the sample and spike the result.
• Mouth alcohol and interference. Dental work, GERD/acid reflux, certain inhalers, and residual mouthwash can corrupt a reading.
• Rising BAC. Alcohol takes time to absorb. If you drank shortly before driving, your true BAC behind the wheel may have been below the limit even though the station test, taken an hour later, read above it. This timing defense can be powerful with an expert.
• Roadside vs. evidentiary tests. The handheld preliminary breath test (PBT) at the roadside is usually less reliable and, in many states, inadmissible to prove guilt, only the calibrated station machine counts.

Blood draws and refusals

Blood tests are Fourth Amendment searches. In Birchfield v. North Dakota, the Court held that police generally need a warrant (or a valid exception) for a blood draw, and a state cannot make it a crime to refuse a warrantless blood test, though it can criminalize refusing a breath test. Missouri v. McNeely held that the natural dissipation of alcohol is not, by itself, an automatic exigent circumstances excuse to skip the warrant. Mitchell v. Wisconsin carved out unconscious drivers. A warrantless, non-consensual blood draw with no exception is a strong suppression target. Note the trade-off: under your state's implied-consent law, refusing the official chemical test usually triggers an administrative license suspension and can be mentioned at trial, even if you ultimately beat the criminal charge.

Challenging a DUI checkpoint

Sobriety checkpoints are a recognized exception to the usual rule that police need individualized suspicion to stop you. In Michigan Department of State Police v. Sitz, the Supreme Court upheld brief, neutral checkpoints. But that approval comes with strings, and those strings are the heart of a DUI checkpoint defense:

• Neutral, written guidelines. Supervisors, not field officers, must set the plan in advance using a neutral formula (for example, stopping every third car), so officers cannot cherry-pick who to stop.
• Purpose limits. A checkpoint's primary purpose must be road safety. In City of Indianapolis v. Edmond, the Court struck down checkpoints whose real purpose was general drug interdiction.
• Reasonable operation. Adequate signage, safety, brief detention, and minimal intrusion all matter.
• State law is often stricter. Some states require advance public notice or published locations, and a dozen states ban DUI checkpoints entirely under their own constitutions. If your state requires notice or neutral guidelines and police skipped them, the whole checkpoint can fall.

If you were singled out at a checkpoint without following the plan, or pulled aside on a hunch, that selective stop may be challengeable just like an ordinary unsupported stop.

When to get a DUI lawyer

Search terms like field sobriety test lawyer and dui checkpoint lawyer exist for a reason: DUI is a technical, document-heavy area where small procedural facts decide outcomes. Hire a lawyer, ideally one who handles DUI specifically, as early as possible, because the administrative license-suspension clock (often 7 to 30 days to request a hearing) starts running immediately and is separate from the criminal case. A lawyer can subpoena calibration logs, get the bodycam, retain a toxicologist for a rising-BAC argument, and file suppression motions on the stop or checkpoint. Even when the facts look bad, defects in procedure routinely lead to reduced charges or dismissal.

Throughout any DUI encounter, your everyday rights still apply: you can decline to answer questions beyond identifying yourself, you can invoke the right to remain silent, and Miranda warnings are required before custodial questioning. Stay calm and polite, do not argue or resist, and save your defenses for court.

This is general legal information, not legal advice. DUI law varies significantly by state, and the right strategy depends on the exact facts of your case. Talk to a licensed DUI attorney in your state.