The check engine lights can be ridiculous, at least on the V70. We have had the same experience...the dealer blamed it on the gas cap. After a lot of return visits, they finally figured it out...it was an emissions control problem (i don't know the name of the part -- it's a charcoal canister designed to trap gas fumes). The light hasn't come on since.

The real problem, in my opinion, seems to be servicers who can't figure it out and then just give you their best guess. I have talked to other owners who heard the same BS from the dealer ("it's the gas cap"). It actually got to the point where I asked if they would simply cut the wire to the light. Of course, the answer was no!