Update - wiring harness has been replaced, but...

[suginami]

An update on the check engine wiring harness on my 1993 300E 3.2. I took my car to the dealer to plead my case on a 'goodwill repair'. Before they committed to helping my out or showing my the door, they checked to see the condition of the engine wiring harness. They checked the number on it and to prove it to me they even sliced open a portion of the black fabric covering on a section next to the firewell, and lo and behold, it has already been replaced.
I guess this is good news, but the bad news is WHY IN THE WORLD DOES MY CHECK ENGINE LIGHT ALWAYS COME ON!!!
The code '5' always comes up, which is an oxygen sensor. The car has a new catalytic coverter and oxygen sensor (changed twice). The mechanic suggested that if the car was running rich it might cause the code to come up. They checked it out while I waited and said it wasn't running rich. He couldn't find any good reason why the light comes on.
The mechanic then loudly proclaimed in a load voice so that everyone within earshot could here, "this car doesn't need a mechanic, it needs an excorsism!" Everybody laughed and I felt like Charlie Brown. You know, like when they go trick or treating, and everyone says what they got, like "I got an apple, I got a candy bar", and Charlie Brown says, "I got a rock..".
As I've said before, I've taken the bulb out so that I don't have to see the check engine light mocking me. But it gripes me to no end that I can't figure out. I know I'm not only anal retentive but also obsessive compulsive.
Does anybody have any suggestions????? Oh, where's the humanity?