The check engine light in my 2000 S40 came on.  When I called the dealer's
service department, the person I talked to said it was probably a loose gas
cap, and when I brought it in that was the diagnosis from the computer.  The
light was reset off, and I was sent on my way.  Two days later, the light
came on again, though I was sure I had tightened the cap when I filled the
gas tank.  This time, the computer software was updated.  The light came on
again the very next morning.  This time, the service manager suggested that
I replace the gas cap.  When he took one out of stock, he remarked that it
was a different design.  I replied that perhaps it was changed because Volvo
knew of a problem with the cap design.

When I was waiitng to talk to the service manager, he was telling another
customer that she seemed to have a loose gas cap.  And when I mentioned
contacting Volvo about the problem, he said something about many others
having already complained.

So far at least, the check engine light hasn't come back on.  Too bad that
wasn't recognized as the problem on the first visit.