Codes P0161 and P1404

[abqchevyman]

Ok, we just purchased a 99 Yukon (old body style with the 350). It has 96,000 miles on it. Before we purchased it, one catalytic convertor was replaced, but not the one on the side with O2 sensors. On the way home, the SES light came on. This happened while climbing home into the mountains at about 6500' elevation. It has remained on, all but about 250-300 miles of cruising at an elevation below 5000'. Not really sure if the elevation is relevant, but thought I would throw it in there.

 

We have codes P0161 (insufficient activity O2 sensor, bank 2) and P1404 (I guess EGR malfunction).

 

I have a hard time believing that the EGR would cause an O2 sensor code and since there are 2 codes, it tells me that something else is the culprit.

 

I've changed the fuel filter (old one was not the best, but not the worst I've ever changed).

 

Any suggestions? I'm leaning to a clogged cat on the side with the O2 sensors and wondering if this would cause excess back pressure on the EGR and thus causing it to throw the EGR code.

 

Thanks in advacne.