Thought I'd fill you in on the DYI option I took on the MAS. It is very easy to do.....but it doesn't make the Check Engine message go away. I took my E320 to the shop, thinking I had just wasted $300.00 putting in the MAS myself, but it turns out, after you replace it you have to reset the computer to take the error codes out. It was confirmed that the MAS was bad based on the 170 code stored, and no current code showing.
They said the check engine message would have probaly gone away by itself after a period of time after the computer "learns" the new readings. I drove about 200 miles, and started the car at least a dozen times before taking it to the shop.....how long does it take before the car figures out the problem is fixed?