To lawfully arrest a motorist for DUI, an officer must have probable cause to believe the person was driving while under the influence of drugs or alcohol. Officers often use field sobriety tests (FSTs) and breathalyzers to make this determination. The breathalyzers officers use to assess whether there's probable cause to arrest a driver for DUI are called "preliminary alcohol screening" (PAS) or "portable breath test" (PBT) devices.
(The more accurate breathalyzers that police use to measure breath alcohol concentration (BAC) after a DUI arrest are called "evidential breath test" (EBT) machines.)
PAS devices are small handheld machines that measure BAC. These portable breathalyzers give police a quick and easy way to estimate how much a motorist has been drinking. But PAS machines typically aren't as reliable or precise as EBT devices.
One main difference between PAS and EBT breathalyzers is the technology they use to measure BAC. Most PAS devices use semiconductor sensors, whereas EBT machines typically measure breath alcohol with more accurate fuel-cell or infrared sensors.
Generally, EBT results are admissible in court to prove a defendant's BAC. On the other hand, many states don't allow prosecutors to use PAS results to prove a driver's BAC.
Without probable cause. Generally, police can't require a driver to take a PAS test without probable cause of driving under the influence.
With probable cause but pre-arrest. The laws of some states require that motorists comply with pre-arrest PAS testing if the officer has probable cause to suspect drunk driving. In other states, including California, drivers are free to refuse an officer's request to take a pre-arrest PAS test. California law additionally requires police to inform drivers of their right to refuse PAS testing.
(Every state has implied consent laws that generally require all motorists arrested for driving under the influence to submit to chemical testing—typically a blood, breath, or urine test.)
DUI laws are complicated and constantly changing. If you've been arrested for driving under the influence, get in touch with an experienced DUI attorney in your area. A local DUI lawyer can explain the laws in your state, analyze the facts of your case, and let you know if there are any possible defenses.