its not an expensive part, if you plan to keep the k5 for while I would change it anyways.
if the o2 sensor is bad it will effect the acceleration and the emissions of a truck
CO possibilities:
Dirty air cleaner or filter (filter looks ok) DONE
contaminated oil (fresh change) DONE
stuck choke You don't have one - it's handled by the injectors. However, a defective O2 sensor might cause a bad air/fuel mix. Check the computer codes.
Carburetor You don't have a carb
mal fuel injection Inspect - check fuel pressure & regulator. Do you smell burning fuel (running rich) or does your truck run hot (over 225 degrees, running lean)?
Thermostat (I just changed for new 6 months ago) Remove and test in a boiling pot of water
cat issues Hard to test, but perhaps look for a date code.
air pump Unless it is seized, it is working.
computer Unlikely, but search for how to display the codes. All you need is a paperclip!
HC possibilities:
Plugs, wires, rotor, cap Inspect!
Vacuum leak Test! Get a vacuum tester.
Exhaust recirculation system Remove, inspect, and test. This WILL fail your smog check.
Spark timing Check this. Borrow/rent a timing gun, search for directions.
Worn valves, seals, guides, piston rings Are you burning oil, blowing white lingering smoke?
Fuel injection system Unlikely this is the problem with your MPG