Can Breathalyzer Tests Be Wrong? Common Causes of False Positive Results
August 13, 2026 | Nafiz M. Ahmed
Yes, breathalyzer tests can be wrong. While law enforcement commonly relies on breath testing during driving under the influence (DUI) investigations, several factors can affect the accuracy of a result.
A high reading can become important evidence in a California DUI case, especially when it shows a blood alcohol concentration (BAC) of 0.08% or higher. However, the number displayed by the device may not always accurately reflect a driver’s BAC.
How Accurate Are Breathalyzer Tests?
Breathalyzers do not directly measure the amount of alcohol in a person’s blood. Instead, they measure alcohol in the breath and use that measurement to estimate BAC.
A properly functioning and correctly administered breath test can provide evidence of a driver’s BAC. However, the reliability of a particular result depends on factors such as proper calibration, maintenance, administration, and the quality of the breath sample.
A problem in any of these areas may call the reported BAC into question.
Common Causes of False Positive Breathalyzer Results
A high breathalyzer reading does not necessarily mean the device malfunctioned. Problems with the breath sample, equipment, testing procedures, or outside substances may also affect a result.
Mouth Alcohol
Breathalyzers are intended to measure alcohol in deep-lung breath. If alcohol remains in the mouth, the device may detect it and produce a reading that is higher than the person’s actual BAC.
Recent drinking, alcohol-containing products, burping, vomiting, or regurgitation may contribute to mouth alcohol shortly before a test.
Improper Calibration or Device Maintenance
Breath-testing devices require proper calibration and maintenance to produce reliable results. If a device has not been properly maintained or fails an accuracy check, its readings may be called into question.
Calibration and maintenance records can help determine whether the equipment was functioning properly when the test was administered.
Testing or Operator Errors
The way a breath test is administered can also affect its reliability. Officers must follow applicable procedures when collecting a breath sample.
Failure to follow required observation or testing procedures may raise questions about the accuracy of the result. Police reports, body-camera footage, and testing records may help establish what occurred before and during the test.
Certain Medical or Physical Conditions
Certain medical conditions may interfere with a breath sample in some circumstances. Gastroesophageal reflux disease (GERD), acid reflux, or regurgitation may introduce alcohol from the stomach into the mouth, potentially affecting the reading.
A medical condition does not automatically make a breath test unreliable. There must be a basis for determining that the condition could have affected the particular sample.
Environmental or Chemical Interference
Exposure to certain chemicals or alcohol-containing substances may interfere with some breath-testing devices. Depending on the circumstances, exposure to solvents, cleaning products, paints, or similar substances may raise questions about the accuracy of a reading.
Whether interference occurred depends on the substance, the equipment, and the circumstances surrounding the test.
How Can a Breathalyzer Result Be Challenged in a DUI Case?
A breathalyzer result may be challenged by examining whether the prosecution can establish that the test was administered properly and produced a reliable result. This requires looking beyond the BAC number itself and reviewing the evidence surrounding the test.
A DUI attorney may examine:
- Records showing the device’s calibration, maintenance, and accuracy checks
- Whether law enforcement followed required testing procedures
- Police reports and body-camera footage for inconsistencies
- The timing of the test in relation to the traffic stop
- Whether circumstances existed that could have affected the breath sample
If the evidence raises questions about the reliability of the test, the defense may challenge the prosecution’s use of the breathalyzer result.
Contact Ahmed & Sukaram, Criminal Defense Attorneys To Schedule a Confidential Consultation With a San Jose DUI Lawyer
A breathalyzer result can be important evidence in a California DUI case, but its accuracy depends on how the test was conducted and whether the equipment and breath sample were reliable. When those factors are in doubt, the reported BAC may be open to challenge.
For more information, please contact our criminal defense law firm of Ahmed & Sukaram, Criminal Defense Attorneys at the nearest location for a consultation.
Ahmed & Sukaram, Criminal Defense Attorneys – San Jose Office
1625 The Alameda, Suite 405, San Jose, CA 95126
(408) 217-8818
Ahmed & Sukaram, Criminal Defense Attorneys – Redwood City Office
600 Allerton St Suite 201G, Redwood City, CA 94063
(650) 299-0500