In response to one question, the car has a 3-way cat. It seems pretty clear to me, after I look at this, that the catalytic converter is bad/defective. The higher CO level of about 1% should actually be decreasing NOX so I can't correlate CO with high NOX. If I were to reduce CO I would expect to increase NOX. So reducing CO doesn't seem to be the solution. The O2 of zero just means that it's running richer rather than leaner though 1% is within Mercedes specifications for the car (may not be stoichometric or however you spell it, but the manual states .5 to 1.5%). I will likely lean the car out more at some point but it's within spec. The car is running very well and the idle is smooth. The HC's are also high given the CO level, which further points to the catalytic converter.

Another post correlated the lack of oxygen with activation in the catalytic converter. I wonder if that necessarily applies here. You could smell the gas in the exchaust, it was definitely working its way through the catalytic converter. The catalytic converter was certainly hot. In the end, to me this adds-up to gas, heat, and a material (catalyst) ready to melt in response to all of this. Whether or not there is an actual chemical reaction around O2 would seem to be an interesting point to understand but I'm not sure if it relates to melting (???)= heat+gas+material

The other cause of high NOX is an engine too hot or one that has carbon deposits. I doubt the engine is running at too high a temperature or has a carbon-cylinder issue. Temperatures seem fine and if it were running so hot, I'd think I'd be having a lean miss and lower CO, none of which is the case. The car's cooling system was recently gone through with a fine tooth comb. I put in a new Behr radiator, new fan clutch, and the water pump is only 2 years old. I've tested the engine temperature with an infrared thermometer and it's right-on.

I'm open to all opinions. The mystery is going to be solved soon since I think I'm going to put another cat on the car.