OK, so my “Service Engine Soon” light came on recently. I tried the gas cap tightening suggestion. It turned off. A few miles down the road it came on again. I had a recall/service schedule (2005 model) so waited until I took it in, however the light turned off (again) before I went in. They did a whole service and (I believe) diagnostic and nothing came up. That was about a month ago and suddenly the morning the light came on again. I have some repairs/maintenance scheduled for end of July (gasket value leak and the right side the axles). The car is driving fine, no difference in feel/sounds.