check engine light says crankshaft timing out of line

1994 volvo 960 2.92L engine -The car runs fine but I had the the service engine light come on, but code fault indicates -crankshaft timing out of line. Is it possible that there is a faulty sensor? If there's a sensor, where is it located? And how big of a job to replace it? If I bring it in to a mechanic how much will he charge? Thanks for your help so I would understand what I'm up against.