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Prosecutors love to present breathalyzer results as if they were handed down from a laboratory sealed in certainty. They put that 0.08 or 0.10 on the screen and expect the jury to treat it as absolute truth. We do not. At Patrick Silva, Attorneys at Law, we know that a breathalyzer machine is a piece of equipment, and like every piece of equipment, it is only as accurate as its last service. When we get our hands on calibration logs, maintenance records, and accuracy check documentation, we regularly find the kind of problems that turn an airtight prosecution into a case full of doubt.
What Breathalyzer Calibration Actually Involves
Calibration is the process of verifying and adjusting a breathalyzer so that it produces accurate readings. During calibration, a known alcohol solution or dry gas standard is introduced into the machine. The device analyzes the sample and displays a result. If the result matches the known value within an acceptable margin of error, the machine passes. If it does not, the machine must be adjusted, retested, or taken out of service.
California law enforcement agencies are required to follow Title 17 of the California Code of Regulations, which sets the standards for chemical testing in DUI cases. Title 17 requires that breath testing instruments be calibrated and accuracy-checked at regular intervals. Each agency has specific protocols regarding how often calibration must occur, who is authorized to perform it, and how results must be documented.
The calibration process typically involves:
- Running a known alcohol standard through the machine and comparing the displayed result to the expected value
- Verifying that the result falls within an acceptable tolerance range, usually plus or minus 0.01 of the target value
- Documenting the date, time, operator, reference standard used, and results
- Performing any necessary adjustments if the machine reads outside the tolerance range
- Certifying the machine as operational for evidentiary testing
When this process is done correctly and documented thoroughly, the machine has a reasonable claim to accuracy. When it is not, everything the machine produced between its last valid calibration and the failed or missing one is suspect.
Your Legal Right to Calibration and Maintenance Records
Here is something prosecutors do not volunteer: you have the right to obtain the calibration and maintenance records for the specific breathalyzer used in your case. This is not a loophole or an obscure technicality. It is a fundamental component of due process.
Under California discovery rules, the defense is entitled to inspect and copy records relevant to the reliability of evidence the prosecution intends to use at trial. Breathalyzer calibration logs, maintenance records, repair histories, and accuracy check results all fall squarely within this category.
We subpoena these records on every DUI case involving a breath test. We demand the full history of the device, not just the most recent calibration. We want to see every accuracy check, every repair, every time the machine was taken out of service, and every time it was returned. This documentation tells a story about the reliability of the machine, and that story is often very different from the one the prosecution wants to tell.
What We Look for in Calibration Logs
When we receive calibration records, we do not just glance at them and move on. We conduct a detailed, line-by-line review looking for specific red flags:
Missed or late calibrations. If the machine was not calibrated according to the required schedule, every result it produced during the gap period is questionable. A machine that was supposed to be calibrated monthly but went six weeks without service has an undocumented period during which its accuracy was unverified.
Results outside the tolerance range. If an accuracy check showed the machine reading 0.09 when the reference standard was 0.08, the machine was reading high. Now apply that same error to your test result. If the machine inflated the reference standard by 0.01, it may have done the same to your breath sample. That single point can mean the difference between a legal BAC and an illegal one.
Inconsistent readings across multiple checks. Calibration logs typically include several test runs. If the results vary significantly from one run to the next, the machine is demonstrating instability. An unstable instrument cannot be trusted to produce reliable evidentiary results.
Incomplete documentation. Missing entries, unsigned logs, undated records, or forms filled out with incomplete information all raise questions about whether the calibration was actually performed correctly. If the person conducting the calibration could not be bothered to complete the paperwork, we question whether they followed the proper procedures either.
Operator qualifications. Not just anyone can calibrate a breath testing device. The person performing the calibration must be trained and certified. If the records show that an unqualified individual performed the calibration, the entire process is invalid, and the machine’s accuracy is unestablished.
Repair history. A machine with a long history of repairs, breakdowns, or recurring issues is inherently less reliable than one with a clean service record. Patterns of malfunction suggest underlying problems that calibration alone may not resolve.
Real-World Calibration Problems We Have Encountered
We are not speaking in hypotheticals. Over our years of defending DUI cases in San Bernardino County, we have encountered calibration issues that would shock anyone who assumes these machines always work correctly.
We have seen machines that went months past their scheduled calibration dates while continuing to be used for evidentiary testing. We have reviewed logs showing accuracy checks where the machine consistently read above the reference standard, meaning it was systematically inflating results. We have found records where the calibration technician used an expired reference solution, rendering the entire calibration meaningless because the baseline itself was unreliable.
In one category of cases, maintenance logs revealed that a specific device had been sent for repair multiple times for the same issue, returned to service each time, and continued producing questionable results. When we presented this pattern to the court, it became clear that the machine’s readings could not be relied upon.
These are not rare anomalies. Calibration failures, documentation gaps, and maintenance shortcomings exist across departments because these machines are used heavily, serviced by overworked technicians, and managed by agencies with limited budgets. The system that is supposed to guarantee accuracy is itself imperfect, and we make sure the jury knows it.
How Calibration Challenges Affect the Prosecution’s Case
When we successfully challenge the calibration history of a breathalyzer, the ripple effect on the prosecution’s case is significant. The breath test result is typically the most persuasive piece of evidence in a DUI trial. Take it away or undermine its credibility, and the prosecution must rely on the officer’s subjective observations: bloodshot eyes, odor of alcohol, performance on field sobriety tests. Those observations are far easier to contest than a hard number on a screen.
If we can show that the machine was overdue for calibration, producing inconsistent results, or maintained by unqualified personnel, we create reasonable doubt about the accuracy of the BAC reading. We do not need to prove the machine was definitely wrong. We need to show that the prosecution cannot prove it was definitely right.
Judges understand that the reliability of scientific evidence depends on the integrity of the instruments and procedures used to produce it. When the calibration logs tell a story of negligence, shortcuts, or systemic problems, even the most prosecution-friendly judge must acknowledge the implications.
Why Most Defense Attorneys Never Request These Records
The uncomfortable truth is that many attorneys handling DUI cases never bother to subpoena calibration logs. They treat the breath test result as an immovable obstacle and focus their defense elsewhere, or worse, they advise their clients to plead guilty because the number looks bad. That approach ignores one of the most powerful tools available in DUI defense.
We request these records in every breath test case because we have seen too many instances where the records reveal problems that change the outcome. It takes effort, time, and the technical knowledge to interpret what the logs mean. That investment is what separates a defense that fights from one that folds.
Patrick Silva, Attorneys at Law Will Hold the Machine Accountable
The state asks you to accept a number produced by a machine as proof that you committed a crime. We ask the state to prove that the machine was working. That is not an unreasonable demand. It is the foundation of reliable evidence. At Patrick Silva, Attorneys at Law, we have the education, the courtroom record, and the technical understanding to make that demand effectively. Our years of DUI defense experience in San Bernardino County and the results we have achieved for our clients demonstrate what happens when the evidence is actually scrutinized.
If you were charged with DUI based on a breath test, do not assume the machine told the truth. Call us at 909-500-4819 and let us pull the records that could change your case.
