An officer may not pull you over without probable cause. Once he or she has a reason to stop your vehicle, then he or she may not enter your vehicle or search it without a warrant or probable cause again. Just stopping you for a traffic offense is not a good enough reason to search your vehicle. The officer may search your vehicle if he or she sees something in the vehicle that you should not have, legally speaking.
For example, if there is a bong on your front seat, the officer may say that there is probable cause to take the drug paraphernalia. In the same way, if someone is screaming in the vehicle, the officer would have a right to search the vehicle to make sure that everything is okay. At the end of the day, there has to be a legitimate reason to search your vehicle at that exact moment if the officer doesn't have a search warrant. In some instances, you may want to allow the officer to search your vehicle willingly, although that's not required and may or may not be in your best interests.
Officers may not arrest you without probable cause. For instance, if you're stopped for a traffic violation and then it's discovered that you're over the legal limit of .08 percent, you may end up with a DUI and be arrested. You'll likely be taken back to the station until you're sober enough to drive or until you're released to another party.
The facts and evidence in your case have to support the officer's arrest. The officer has to show that the same knowledge given to another person would also make that person sure that you were committing, about to commit or had already committed a crime. A hunch is not good enough; there has to be evidence for an officer to arrest you. Our site has more on what you need to know about traffic stops and drug offense defenses.