What are you, some type of amateur legal beagle? First off, no probably cause is needed. This is an administrative inspection (a warrantless search) whereby they are merely looking to see if a vehicle and it's driver are in compliance with the vehicular laws of the state (license, insurance, inspection, etc.).
The probable cause part comes when the law enforcement officer observes patterns of behavior which indicate possible violations of criminal code.
So, amateur lawyer...go back to school and review warrantless searches (there are at least 7), reasonable suspicion, and probable cause. Be sure you also understand professional and logical inferences, plain and open view doctrines...and all sorts of other things.
Driving is indeed not a guaranteed right.
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