Everyone is likely headed into the time frame to replace 02 (AKA Oxygen Sensors). Recently my engine light came on at 60K miles....strange because car was running fine. I never looked under my H3 Alpha at the unbeleivably horrific exhaust system but was amazed that not only was there two 02 sensors but there are two ON EACH SIDE.....Of course this is for the 5.3, I am not sure for the 5 cylinder. Anyway, I put it on a OBD scanner and to my surprise I see 02 sensor 1/1, 1/2, 2/1, 2/2...here is a hint for your problem...front 02 sensors rarely go bad. so you are likely going to need one after the cat. This might save you some time and trouble of replacing all at $50 a pop. THE PLUGS ARE DIFFERENT ON FRONT & REAR

1/1 - Upstream (Passenger side front) Before converter
1/2 - Downstream (Passenger side rear) After converter
2/1 - Upstream (Drivers side front) Before converter
2/2 - Downstream (Drivers side rear) After converter

These are relatively easy to change and test since they are mounted on the inside of wheel well where rain, mud, snow constantly contaminate the plugs.

Heat up your car a bit....get a 3/4 wrench and loosen.
Use a volt meter on .0 DC
Connect black to metal surface (the nut) of the 02 Sensor
place red lead on black wire
put sensor into flame of a propane torch
voltage should rise immediately when placed in flame and fall when removed from flame. This is not a 100% test but I have never had it fail.

Check the internet for pricing....you should be able to get the DIRECT REPLACEMENT FIT for about $50 shipped.

You can save a few bucks on the UNIVERSAL FIT at your local parts store if you want to go through the hassles of soldering/crimping and fearing a loose connection.

Most local parts store can hook up your car for free and tell you which code and what is the problem or find out enough you can google the code for your answer.

Hope this helps.
JJ