I have been trying to get my 2004 E320 wagon to pass smog. I first took it in, and the tech told me it has no CEL. I only bought the car a few days ago, I didn't notice it being out (I am a Diesel guy, and didn't even look for it). I have another car (sedan), so I pulled the cluster out and swapped it. The CEL worked, but the odometer showed - - - - - - -. No problem, it only needed the CEL to work. Back to the smog station, it now has two IM monitors that won't set. I started this ththread:
http://www.peachparts.com/shopforum/tech-help/394453-2004-e320-drive-cycle-secondary-air-monitor-not-ready.html
Now, I decided to try and find out how to replace the CEL bulb in the wagon cluster. I disassembled the cluster and removed the circuit board with the clock in it. Much to my surprise, there are no replaceable bulbs, but LED's. I then looked through the window where the CEL should be, and it was all black with no "check engine" shining through. I pulled the circuit board off the other one (sedan), and I could clearly see the check engine through the window. I thought: how could this be? It has to have a CEL! I got a flashlight and looked down inside, and saw something odd. I got a pick and picked out a piece of what looked like black carpet. I have to think that someone put it in there, and it was not there from the factory. I put everything back together, and put both clusters in their respective cars. Both work as they should- Hooray!