And until you get a reader, first thing I'd check is all your fluid levels. In my instance it was low coolant level. I never saw that temperature gauge climb to anything abnormal, but... it would have if I'd let it go and not noticed that I was losing coolant in a big way. If you just changed the oil, perhaps your oil level is a bit low (enough to trigger the sensor)??
Well after talking to several people about this problem I decided to replace the new fuel filter I had just installed with another brand and the light went just like that.Wow was I relived when the check engine light went off. So my problem was a defective fuel filter. I had estimates anywhere from 100.00 to 200.00 to run a scan before I found the bad filter.
A Scangauge for diesels is in the works, but has not been released yet. He will have to take it to a Cummins dealer to get the computer read, or buy a Silverleaf Reader.
If you goto the first link I posted, it says that the ScangaugeD is available now at Autozone, CampingWorld, and Canadian Tire. I have not yet tried to get one myself, so I can't say how hard/difficult it is. It did show up in the available inventory for CampingWorld though when I checked for it online....