SES light mystery

So I'm back after several months with news on my 2006 325i. Long and short of it. SES light referencing O2 sensor, replaced both pre cat sensors. SES on again same coding. Problem almost reproducible. And so on. Went in to the dealer. Replaced air oil separator. Light on again same codes as before much frustration on their part replaced the DME at no charge to us. SES on again same codes as before Dealer replaces body harness no part cost but a reduced labor cost. They have been realy great about fixing trying to fix the problem with many weeks in a loaner car and lots of good will and after all this the dealer still seems uncertain weather it is fixed. What else could there be in the system to go wrong?