Deputies said some of the readings were so high, they would have meant instant death.
The district attorney's office sent memos to attorneys involved in cases that included blood-alcohol tests taken on eight erratic machines between Jan. 20 and March 31.
The county, which bought 125 of the Intoximeter Alco-Sensor V devices at $4,800 each, has shipped them back to the manufacturer.
"The ones already found guilty by trial or pled guilty, we are going through all the cases whether we would be withdrawing pleas," Chief Deputy Public Defender Monica Cummins said.
Cummins, who represents 160 people arrested with the faulty devices, confirmed that many had pleaded guilty after seeing the results.
The maker, Intoximeter of St. Louis, is trying to correct the flaw.
The Ventura County Sheriff's Department expects to have the retrofitted devices back in three months. All will be tested for 100 percent accuracy before they are used again.
The Associated Press contributed to this report.
