Could mean a lot of different things. You need to get it read by a diagnostic tool. You can buy one and do it yourself or get raped for it at the dealer. Some dealers will check them for free but I haven't heard of any good dealers in the San Jose area. I'm from Morgan Hill. I can only recommend Tiffany Ford in Hollister right at the end of highway 25 once you get into town. Or you can post your question on norcaldieselforum.com and there is a tech there that posts under the screen name strokin early 99 but I am unsure what dealership he works at. He could help you out.