I was having an un-explained EGR problem, with P0171 and P0174 codes as well. Cleaned out everything EGR related (there was
no soot in there) and still had the codes.
Took someone's advice and had a look at the MAF Sensor, sure enough, there was a hair and plenty of crud on the wires. Cleaned the wires, re-set the OBDii system, and viola.. no more problems with the CEL.
