Helping a friend diagnose his 2003 Chevy truck 5.3L

PO171, 174 Too lean.
PO300 missfire.

The post cat O2 sensors read very different from each other. One moves up and down in voltage like the pre-cat sensor the other puts out a more steady .7 or .6 volts.

The long term fuel trim on one bank is over 23%, this tripps the CEL.

I didn't think think the post cat O2 sensors affected mixure??

Or is it a clogged converter and the sensor is working correctly, telling me I need a new converter??

The truck stumbles and wanders at idle.

A good puzzle I think.