Here is what the shop order from a MB dealer for my car when I took it in for the check engine problem:
Follows is the excact write up:
Code 004. Air Injection. Ports for air injection plugged with carbon. checked for codes. code 004. checked to see if air injection valve has vaccum. it does. check activations. The O2 voltage should drop to below 80mV. It does not drop. See recommendations.
Recommendations: Need to R&R both Heads and exhaust Manifolds need to clean air injection port $2600.
Engine harness cracked $800.00.
When I took it to the independent tech he resetted the check engine light after using his diagnostic computer to read a generic diagnostic like "Air Injection pump faulty". I drove the car for about 100 miles and the check engine light came back on. At that time, he removed the speedo cluster and removed the bulb under the check engine square. He also told me that the car will pass smog without any problem since the air injector only works during cold start. BTW, The O2 sensor were replaced by the dealer 10K miles prior this. Apparently, the check engine light has been coming on and off prior to my purchase of the car. The PO has been taking it to the dealer and from the records that i have the dealer did almost everything you have gone through.