My 97 4.6L CEL came on....insufficient EGR flow. Vacuum checked the EGR valve, OK. Checked EGR tube and hoses, all clean and clear. Pulled the intake goose-neck to unclog the passages and the passages were blackened, but not clogged. Ultimately replaced the DP sensor and all is now well.
The EGR ports are so dam*ed big I don't see how they could ever clog, but some do I guess.
I've been running my 94 LTC with a CEL caused by insufficient EGR flow. That car DOES have some EGR passage clogging problems, but getting at the passages is a PITA. Florida does not require vehicle inspection so I'm in no hurry to correct the problem. Fuel mileage has not changed at all, much to my surprise.