Rep Power: 0

Had the CEL come on a few weeks ago and had to drive the car back and forth to work. Installed parts until the primary codes went away, but now I've got the P0420

Searching facebook and craigs for a used CAT, but I just want the damn light to turn off.

In the next 50,000 miles, would a non-fouler cause the computer to make the fuel more rich or leaner?

The cat is a bolt-on, bolt-off job, but until I can find a replacement, I don't want to cause more problems

 
Rep Power: 511

if you did it to the upstream sensor, yeah, it'll likely make it run more rich, since it'll be sensing more oxygen in the air/fuel ratio, but that won't do anything for the P0420 code.

The downstream sensor is the target sensor for this trick, and its purpose is to monitor cat efficiency. I ran with the non-fouler trick in my EM2 for... probably 70-80k miles with no AFR-related issues. I had a wideband AFR gauge installed, so I could tell you my AFRs were sitting happy the entire time (14.7 ± 0.8ish for normal driving, expectedly rich at 11.9 at WOT)

I had a high flow cat, that's why mine threw a P0420 code. Only reason I replaced my cat with another high flow cat (and still did the same non-fouler trick) is because I apparently ran something over or bottomed out hard enough and there was a softball sized dent on the bottom side of the cat itself.

 
Rep Power: 222

Defouler on the secondary (where it's supposed to go) won't cause any issues. That sensor is only there to monitor the cat.