DTCs will . . .
NOT erase themselves in four (4) hours but will be removed from 'memory' after a number of 'trips'. A trip is defined differently for different year/models but in general, it's when:
1) the car runs for more than 5 minutes
2) speed is greater than 15mph
3) and a bunch of things I have forgotten!
So after 5 trips, the codes are removed if they do not occur again! Not all of the needed parameters but you get the idea.
Your car is exempt from the Dyno test since the ASR is NOT defeatable. My '94 S500 has the same ASR and it gets tested w/o the dyno.
Your SMOG station should have known that because of the non-defeatable ASR. In later models, the ASR was (is) defeatable.
I would demand that they test you for free since it's their error.
Regards . . . . JimF
'94 S500 Cpe
Visit my Mercedes Web Page