P-0401 - exhaust gas recirculation flow insufficient detected. Your clue in this one might be the word "detected". It could be several things.
1) possibly the EGR sensor is going bad
2) the EGR valve is controlled by a vacuum system, so you may have insufficient vacuum to the valve or you may have a ruptured valve diaphragm causing it to not open very far thus sending a code to the computer via the sensor
3) highly unlikely, but it could actually be a partially plugged EGR passageway leading up to the valve from the exhuast system. Highly unlikely because that is something the vechicle's computer would be hard pressed to detect

Answered by Indyuke
2 questions asked

Thanks for the reply, The EGR valve was replaced with a factory part, as was the DPFE and the EVR (vacuum control solenoid) these three parts are the only ones shown in the block diagram of the EGR system. I will pull the EGR valve and check for obstructions going into the intake manifold (which I haven't done yet) and hope that takes care of the problem.

Response from pkjfs
3 questions asked