I've enjoyed being a lurker in these forums for a while now, but a recent issue has cropped up on my wife's 2001 ML320 that I need input on.

A few times over the past year, the truck wouldn't start up; it would turn over but not catch/start. Waiting at least 15 minutes and trying again would do the trick - the engine would fire right up. The check engine light never comes on, and on previous occasions the dealer has not been able to find the problem; I assume that no error codes were reported by the engine.

This afternoon, after filling up at the gas station, the truck refused to start again. However, this time the engine would turn over and almost catch (causing the truck to shake slightly), but then fail. A friendly mechanic driving by looked at it and thought that the mass air sensor needed to be replaced, noting that he's seen that particular problem often. After trying for an hour and a half, we called AAA for a tow - only to have the truck start fine once the tow truck arrived .

From what I've read in these forums, it seems like the crankshaft position sensor is the culprit; if so, I'll be ordering one and attempting it myself. However, the mechanic's advice about the mass air sensor makes me wonder - does the mass air sensor only need to be suspected if the check engine light is on? Is there any other things I can try to help narrow down and troubleshoot the issue?

Thanks in advance for any input you all can provide.