I wanted to update you on this puzzling situation. I replaced the left rear O2 sensor, cleared the 2 codes (161, 173) by disconnecting the battery and after 10 driving cycles (the vehicle re-learned the proper fuel mixture ? per DVD), CEL is back on with the same 2 codes. Has anybody seen the same two codes and CEL in '99 ML320 (mine has only 66k) or in any MB?
Also, the auxiliary water pump stopped working - shoud run 10 mins after the engine shut-down, but all quiet on the western front.....I measured the voltage in the connector anf it is definitely getting 12.6V minutes after shut-off. Could these 2 be related i don't think so, but wanted to ask anyway.

Please, please help, I am getting desperate here

Thanks in advance.