As a general rule, the results of the PBT cannot be used against you at your DUI trial.
If you're charged with a DUI, the preliminary breath test (PBT) cannot be used against you at trial. A PBT can be used against you if you're prosecuted for offenses other than DUI, for example public intoxication. But that doesn't mean it's a good idea to submit a preliminary breath test. The reason a PBT isn't admissible in your DUI trial is because they aren't proven to be very accurate. That is the best reason not to submit to a PBT.
If a PBT is not admissible against me at my DUI trial, why is the officer asking me to submit to one?
The first step for the officer is to gather enough evidence to place you under arrest. Before a police officer can place someone under arrest, an officer needs probable cause. Probable cause means enough evidence to make is probable you have committed a crime. The results of the PBT can be used by the officer to prove he had probable cause to arrest you. Because probable cause is a much lower burden than beyond a reasonable doubt--the burden at trial--the PBT can be admissible in some limited circumstances.
If a PBT can't be used against me at my DUI trial, then why shouldn't I submit a PBT?
That's because there are exceptions to the rule that a PBT is not admissible in your DUI trial. For example, if you challenge not only your innonence, but also allege that the officer didn't even have probable cause to arrest you, judges will often allow the police officer to testify about the results of the PBT. Most judges, if they hear from the officer that you had alcohol in your system, will find that the officer had probable cause to arrest you for DUI. So, if you want to challenge whether the officer had probable cause to arrest you, you should not submit to a PBT.