I was getting check engine light for sometime, on my 92 5.0 4X4 My local shop (who always seemed to know what they were doing) could not figure the cause. As the truck has 185K miles on it, they suspected the catalytic converters. It did run quite rough in our hot SC summer. So I bit the bullet and had one taken out, and replaced the other. It runs better, but sill check engine light on. I decided to watch the videos in Youtube on reading the codes. There are one or two excellent ones on there. I checked mine and was getting the code 33. Reading a bunch of posts, this is a pretty common problem, as any number of problems can cause it. I checked out the EGR vacuum - ok, cleaned it and all seemed in good working order. Vacuum lines next. They had been repaired before, so replaced some of the old "fixes" I then checked the EGR vacuum solenoid. There did not seem to be much movement when hooked up to power, but I don't think a new one "moves" that much. So I replaced the old one with a brand new "gray" top one. Be careful as there are two types, so make sure you get the proper one for your truck. And walla - NO check engine light! Reading some others posts, it seems like some owners just start throwing money at the problem without trying to eliminate each component in turn . These vacuum solenoids do a lot of work, and seem to wear out over time. Truck runs great now. I also used to get a lot of pinking when it was really hot, this has also been fixed with the new solenoid.