State v. Cassidy – New Jersey DWI Breath Test Case Law Explained
October 1, 2026
In 2018, the New Jersey Supreme Court decided one of the most consequential DWI cases in state history: State v. Eileen Cassidy, 235 N.J. 482 (2018). The unanimous decision threw out more than 20,000 DWI convictions based on breath tests from improperly calibrated Alcotest machines.
What happened
New Jersey has used the Alcotest breath-test machine since 2000. In State v. Chun (2008), the Supreme Court approved the machine’s calibration protocol — but only if strict procedures were followed, including checking simulator-solution temperatures with a thermometer traceable to National Institute of Standards and Technology (NIST) standards.
Sergeant Marc W. Dennis was responsible for calibrating Alcotest machines across five counties: Middlesex, Monmouth, Ocean, Somerset, and Union. Investigators discovered he had skipped the required NIST-traceable thermometer step and allegedly falsified the records to cover it up. He was indicted in 2016.
The legal question
The State asked the Supreme Court to decide the issue directly: are breath-test results admissible when the Alcotest wasn’t calibrated with a NIST-traceable thermometer? The Court appointed retired Judge Joseph F. Lisa as Special Master. After an extensive evidentiary hearing, his 198-page report concluded that accurate temperature readings are the foundation of the entire calibration process — without them, the machine “learns” an incorrect standard and every reading it produces is suspect.
The holding
The Court adopted the Special Master’s findings: breath test results from Alcotest machines not calibrated using a NIST-traceable thermometer are inadmissible. The Court ordered the State to notify affected defendants and relaxed the five-year time bar so people with old convictions could seek relief. In total, 20,667 DWI convictions were tainted.
Eileen Cassidy herself had pleaded guilty based solely on an Alcotest reading. When the calibration scandal broke, she moved to withdraw her plea. She passed away before the case was resolved, but the Court vacated her conviction.
Why it matters
Cassidy is the companion to Chun: Chun says the Alcotest is reliable only when the protocol is followed to the letter; Cassidy shows what happens when it isn’t. For anyone charged with DWI, the lesson is the same — the breath test is not automatic. Calibration records, thermometer logs, and maintenance documents can all be challenged, and when the State’s science doesn’t hold up, cases fall apart.
If you were convicted of DWI between 2008 and 2016 based on a breath test in Middlesex, Monmouth, Ocean, Somerset, or Union County, you may be entitled to relief. Call (732) 303-7341 for a free consultation.
General information only; not legal advice. Every case is different.
