Alright guys, wondering if you could help me out here. First off, my car is a 2000 Formula a4, 3.23's. with 62k on the clock. Only mods are lid/cutout, and a slowmaster.

Alright, I have a feeling that this can go a few ways...

So, when I first bought this car I compared it to my '99 a4 GU5 T/A, and stock it felt a little faster than my T/A did with lid/cutout (13.6 @ 104) So I went ahead and installed a lid and cutout on the Formula a few weeks later, and everything was going well. One day while driving I get an SES and it stays on for a few hours and then disappears, it didn't come back so I thought nothing of it.

I end up buying some chrome 10 spokes from a guy in my area and throw them on, moving from 245/50/16''s on twisties all around, to 275/40/17's all around. Now the car starts feeling a little slower, but i figure the wheel/tire combination might have put a little weight on, so it was no biggie.

That weekend I go out to have a quick run with a couple of friends who also have F-cars, one is a 98z and the other is an 01 Formula. Now, to me, this car felt a good deal faster than my sloppy shifting T/A did, and the T/A would put a good car or car 1/2 on my buddies Formula, and the Z was about on par with my friends Formula, so I expected a decent outcome for me.

I was dead wrong. While driving to the spot my SES comes on again and I get a little paranoid, but I still wanted to see how it matched up against my friends cars. The Z ended up putting about 2 cars on me from a dig, and all he has is a lid and catback. Then my friends Formula put a good car 1/2 on that Z, leaving me feeling like a total ***...

I check the codes and I'm getting p0420, bad cat or o2. So I do what I can do for free and take it down to the stealership and have them replace the cat. Got it back and the car felt better and the ses was gone. Had another run with my buddy in his Formula and we were about even from a 25 roll, my confidence in my car was back.

Now, here's my question. That SES light came back again, and it's p0420, which leads me to believe I have a bad o2. I decided to see how I matched up against those same cars, and from a dig they each put about 1 1/2 cars on me off the launch because my car just refuses to launch, and bogs no matter how high I rev it out. From a 25 roll however, things are different, these same cars that would put buses on me are now right in my passenger window the whole time.

Could the bogging off the launch be from a bad 02, the heavy wheel/tire combo, or both? Maybe something else? What should I check? The tires will not even come close to breaking loose, even if I launch just at the point where the tires feel like they're gonna spin, it'll still bog. I think my 60's are probably in the 2.5 range... My T/A was never this bad.

Oh, and it feels exactly the way my friends M6 SS felt when he'd accidentally leave his TCS on, mine doesn't have TCS though.

Sorry for the book, but I wanted to get as much info as I could. Thanks guys.

 

A rear o2 sensor being bad wont cause any performance loss. It is only there to check up on the cat(s).

Maybe time for a tune up? Check your spark plugs/wires. And replace the fuel filter. Might have gotten some bad gas. Thats all I can think of bud.

Josh.