The relationship between policing and automobiles is long and complicated. Law enforcement’s ability to stop and search a vehicle comprises a distinct line of Fourth Amendment jurisprudence. But searching a vehicle no longer means what it did even twenty years ago. Today, automobiles collect data on us from when we open the car door to the moment we turn off the engine. Much of this information is retained in an automobile’s hardware and funneled to third party companies, who can share at their discretion. Law enforcement agencies have made use of auto data, obtaining it without a warrant both by extracting auto data from the vehicle itself and by contacting the companies collecting the data firsthand to ask that they share the information. The constitutionality of such a practice may seem up for debate, given the disagreements among lower courts over how auto data fits into a larger web of Fourth Amendment jurisprudence. This Note brings together two strands of Fourth Amendment case law—the automobile exception and the third-party doctrine—and argues that an animating principle motivating the Supreme Court’s recent digital search cases provides a clear answer to the auto data confusion: Police need a warrant before they can access auto data, because auto data, much like cellphones and cell site location information, reveals automatically collected diaristic information.