The reason I ask is because these cars are VERY picky when it comes to Cats. Sometimes replacing the stock one will set the P0420 code, even if your O2 sensor and Cat are fine. So if you still have the code and changed everything, it might just be that the car doesn't like that cat.

 

If the replacement cat has a higher flow rate than the OE cat did it's possible that is setting the code. You might eliminate the code by using the spark plug anti-fouler trick. Get an anti-fouler then drill out the center of it so the O2 sensor will fit inside. Thread the anti-fouler into the O2 bung on the downpipe, then thread the O2 sensor into the anti-fouler. This will move the O2 sensor further away from the exhaust stream and hopefully stop it from triggering the code. I was going to do this to mine when I replaced my downpipe/cat but so far mine hasn't thrown a code.
I test fit the Dorman 15/16" (18mm) gasket seat anti-fouler (part #42009) on mine and it sealed against the O2 bung nicely.

 

I can definitely verify this happens. My dad has an '02 LS with an aftermarket cat. It throws the P0420 code constantly. I should try the anti-fouler trick, but I haven't had much time to do more than the bare necessities in the last year.

 

If this is the case, is there any reason why I would be seeing a 2-3 MPG average DECREASE in mileage? This is on the mileage on the computer of course, but I am in the process of checking the MPG the realiable way. I was told by someone one time that the rear O2 sensor shouldn't affect mileage. Maybe there is something else wrong as well. Kind of frustrating.

 

There are a lot of reasons why your MPGs can change. You mention a u-bend delete. Was the cat changed, or was the WHOLE downpipe changed? Some of those aftermarket downpipes with a cat attached are even MORE restrictive than the stock one.

 

I had to get a new cat two years ago, so I got the U-bend deleted at the same time (no change in downpipe). Two years later, another bad cat. This is the Fourth Cat on the vehicle. Original one was swapped by original owner (aftermarket not covered by GM recall = dumbass). So when I got the car it was on the 2nd cat which was after market so when I tried to get it fixed under the recall I was charged $75.00 for chevy to read the code and tell me it wasn't an OEM cat, so I had to pay for the "read" and a new cat. Got the cat replace (my first time, cars 3rd for $270 with U-bend delete from my exhaust guy). About three months ago I paid another $250 to get a new cat installed. Rather annoyed with the cat issues if you catch my drift. But anyway I would like to see my 26-30 mpg average again....Recently had a tune up which included: plugs, wires, and rear O2 replaced. Havent checked the air filter, but I need to and it probably needs swapped out. Haven't decided to get a drop in or CAI system (probably Wizaired if I do).

 

You might need to get some diagnostics run. You could be running really rich for some reason (Bad maf, off tune, open injector) because running rich can kill a Cat Converter really quick.

Fun fact, a dirty air filter will actually help your MPGs a little bit haha. Less air = Less fuel being dumped at that air. You just feel less power. (of course that is assuming you aren't using the throttle more to compensate)

 

On his car there has been no decrease in gas mileage. No symptoms of any kind, actually. Just the P0420 code.