With reference to egr I believe if this sticks open idle you could possibly get slight misfire you may not get a fault code for egr when faulty as although there is a fairly big plug it only has 2 pins I recently had a focus in for blue smoking and that was being caused by the egr valve ( I know it's a different system but principal is the same) I would be inclined to blank the egr valve off and let your customer try it and if all ok for a few weeks then replace egr as they are not nice to do.
HTH
Carl