Police officers can impound your car for a variety of reasons. If you are arrested for a traffic violation, like a DUI, and no one else is present and able to drive your car, then they will typically impound it. Illegally parking or abandoning your vehicle also risks impoundment. And once officers have legally impounded your vehicle they are entitled to search it.
Police officers are allowed to search an impounded vehicle to conduct an "inventory search." An inventory search doesn't require a warrant or probable cause, because it isn't supposed to be a search for evidence of a crime. Courts have upheld inventory searches on the theory that the police should be able to search the car to protect the owner from theft of any items in it, and to protect themselves from any claims of theft. (Officers can prove what was and was not in the vehicle when they took control of it by creating an inventory.) Another reason courts have allowed inventory searches is police protection: Officers should be free to search for anything in the car that could endanger them, such as weapons or explosives.
How thoroughly can the police search an impounded car? It depends on the jurisdiction. In some states police officers can search every nook and cranny of the car, while in others a brief sweep for items is all that's permitted. In almost all states officers can take inventory of any object in plain view—that is, any object readily apparent to the naked eye.
If the police impound your car and find evidence of a crime in it, you may want to contest the search; if you win, the evidence won't be admissible in court. But, winning this argument can be tough. You will have to prove that the officers acted unreasonably or in bad faith, such as by purposely breaking the law to impound or search your vehicle. Honest mistakes by officers aren't enough to win an illegal inventory search argument.
The following are situations in which courts have found inventory searches illegal: