Many courts will hold that license plates are public information and the police can check them.
As to probable cause, the police have the right to pull over a vehicle for a routine traffic stop, but traffic violations by themselves do not give rise to probable cause. The police must have cause to believe that the persons in the vehicle are concealing evidence or a weapon to do the officer harm. Examples include suspicious movement by the persons, spotting actual evidence, or getting the person's consent for a search.
Please speak with a criminal defense attorney for legal advice. I am not a lawyer.
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