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P0403 is a circuit fault. If the concern and the code were not present before you replaced the EGR valve it is suspected that you have a defective new valve. This code if set with other EGR related codes might be caused by carbon getting caught in the valve. You mentioned there was a lot of carbon - unless completely cleaned out loose carbon can find it's way into the valve and cause position sensor codes and driveability concerns like those you mentioned. What did your diesel shop come up with?
