Heads up for anyone who may go down this road...
It would seem there is an error in this document:
http://www.geocities.com/MotorCity/Downs/1039/mercedes/cs1000_mb.pdf which was referenced earlier. It is a big 169 page pdf file listing error codes...it's actually a manual that came with a code reader.
I have a CODE 31 which that document says is an intermittent coolant temp sensor circuit.
But now I notice that it says that both error code 28 AND 31 are a coolant sensor circuit problem. Obviously, that cannot be true.
The factory documentation says code 28 is a coolant sensor circuit problem, and 31 is an intake
air temperature sensor circuit problem. However, there is plenty of confusion supplied in the factory documentation, too, because in the repair section they twice refer to air temp sensor as the coolant temp sensor, even though they call it by the correct part number.
So I've been chasing a coolant temp sensor circuit problem when, evidently, my problem is with the air intake temp sensor circuit.