I don't know about a "direct fit", which I'm assuming is one that you can bolt in yourself. I had a high flow CAT installed (welded) in my '03X by my trusted muffler shop for around $200. The procedure is to cut out the old CAT & weld in the new. This still leaves the existing flange, so you could remove it. :smile:
I had to look over the shop manual for the '99. My '03 & '07 only has one CAT, but I see your '99 has 2. :icon_eek: The CAT would be required to pass emissions. Yes, you could run with one, but it may not pass emission testing. As to the engine, it'll run fine with one, two or none (not recommended). :smile:
As to changing the O2 sensor. Which one, front or rear?
If you're running rich, your front O2 sensor (Air/Fuel) may be failing. The P0420 is reported by the rear O2 sensor detecting too much O2 passing thru the CAT. I'm not a mechanic, so this is just my guess. :wink:
A P0420 can also be caused by an exhaust leak ahead of the rear O2 sensor, but this wouldn't cause your engine to run rich. :smile:
No, you don't want to mess with the front O2 sensor! It's critical for correct engine operation. Yes, bank one would be the front O2 sensor & I'd recommend replacing that first, before the CAT or the rear O2 sensor. In addition, I'd recommend getting a factory/OEM replacement for the front sensor. Like I said, that sensor is one you don't want to go cheap on. :icon_eek:
Just for information, the non-fouler spacer is only for the rear O2 sensor. It works by moving the sensor back out of the exhaust flow. I'm thinking once you replace the front O2 sensor, you won't have to worry about this. :wink: