If you remember, Walt drove Hank to the "Industrial Laundry" that Hank had sussed out as consuming way-y-y too much energy and had these industrial air filtration systems from Germany. So he had a suspicion it was housing the meth lab - WHICH IT WAS.
Walt insinuated himself into driving him, using his Aztek to get them there, and getting more and more nervous as they approached. He pretended to be lost as they approached, missed the driveway, then passed the facility, requiring him to U-turn. He does this intentionally right into an oncoming car and BAM! totals the Aztek.
So, Hank doesn't get into the laundry because he stays with Walt, who's all-too-flustered from causing an "accident". Also, we see Hank later in a neck brace, presumably injured in the "accident"; maybe there was an ambulance at the scene (I don't remember it)?
All this happens in the episode Crawlspace, the one where Walt finds most of the money missing, because Skyler gave it to Ted to pay the IRS.