Afraid I'm not much help besides what I've already suggested. The EGR valve itself is really simple - Just a vacuum diaphragm (follow the tube from the transducer) that pushes or pulls a little plunger to open a passage for the exhaust to flow into the EGR housing, which is bolted to the bottom of the intake manifold. I suppose the valve could be stuck open but I don't know for a fact that this would cause a code. There's a huge difference in performance when our cars are in limp mode but they've both been chipped so maybe the difference - limp versus normal operation - is greater. With the electronics defeated we gutted the EGR housings, plugged the holes, and removed the EGR pipe that runs around the back of the engine from the exhaust manifold to the EGR housing. Once you've had the manifold off you'll be an expert and find you can r/r it in no time. I suggest you soak all the glow plugs with PB blaster or similar while you've got access. Anything to help avoid having one stick/break is a good thing...trust me !

Q #1 - You could disconnect the charge air pipe form below and look up inside with a light. Wont be very rewarding.

Search Po400 too = P oh four zero zero

After you pull the IM you will probably find everything is pretty gunked up with greasy soot. Took me an hour to take things apart and 6 hours to clean it. i wouldn't mess with your GP's unless one is bad. Why tempt fate and snap one off that works?

__________________
Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)