My car had done only about 37K when the check engine light came on so a
failed sensor was unlikely. Code PO170 didn't describe the exact problem
so I paid the dealer to diagnose and estimate the repair and its cost -
MAS about $550 - $600. (On line part price $260.)
Then I thought about the project and decided to first try cleaning the
MAS and replace it DIY if cleaning failed.

My E320's MAS has two pieces: the larger 3" x 4" "stovepipe" housing and
the sensor itself which screws into the side of the "stovepipe".

Removing the "stovepipe" is easy. Then one removes the sensor from the
"stovepipe".

I used an old fashioned (smoker's) pipe cleaner to simply clean the
sensor's wires - very, very carefully, in daylight.

I didn't use any solvents for there are no petroleum deposits on the sensor.

Cleaning worked, there's been no further messages.

I cleared the codes with my code reader.

I did nothing to clear the light.  Just drove it and it cleared itself in a
few days (I drive > 120 mi/day.)  You can reset the computer by
disconnecting the battery, just make sure you have your radio code handy.

I used carb&choke cleaner to clean it, and the whole job took about 20
minutes.  Just be sure not to touch any of the sensor components.