Hello. I have recently purchased a 2000 S40 1.9t with 50000 miles on it. I bought it a little over a week ago and two days after I brought it home the check engine light came on. (I did make the mistake of putting 87 octane gas in on the way home. But 91 octane ever since the intial fill up.) So I took it back to the dealer and on the way back the light went off. So they hooked it up to the computer and found nothing. They did a carbon cleaning. They drove it around a little and then I got it back yesterday and took it on a small roadtrip no check engine light. Then I go to leave this morning and the light came back on. And has been on ever since. I don't really know where to start any ideas would be greatly appreciated. Thanks.