As Volkswagen AG prepares to explain how it might fix the polluting diesel cars that set off the biggest scandal in the German automaker’s history, whatever solution it proposes could be undermined by some owners in the United States who won’t bring their cars in for a recall and can’t be forced to. In the three states with the most VW diesel cars caught up in the emissions scandal, only California withholds registration renewals from cars that fail to comply with recalls.