The Check Engine light came on in my 2000 S40.  I went to the dealer's
service, where they checked the computer and said it was because the gas
tank cap wasn't put on tightly after I got gas.  In a few days, the
light came back on.  They reset it again, after finding nothing wrong.
The next day, the light was on again, though I hadn't touched the gas
cap.  The service manager got involved (I insisted on it by now), and
suggested trying a new gas cap.  He took one out of stock, and remarked
that the design was changed.  It's a couple of months since the new cap
was put on, with no Check Engine lit.  I guess Volvo changed the design
for a reason and isn't being open about it, not wanting to pay for
replacing a lot of caps.