I have tried a couple of scan tools, but none have much of anything in the way of live data on the Benz.

It is definitely the self test that is failing - the fault can only occur when I'm idling and not driving. I have a 40 mile interstate commute through Atlanta traffic, so I make sure its clear before getting on the highway. If I keep moving I'm good for the trip. If I get stuck in traffic I keep tapping the accelerator as that prevents the self diagnostic mode from starting. I would love to know a combination of sensor inputs that would simply prevent it from self testing - maybe coolant temp too low or something like that?

Oh, its a 1997, btw.