Another vote in favor of non-foulers on an O2 sensor
 

Hey guys, just like the title says, I'm now firmly in the team that says use spark plug non-foulers to clear out a P0420 after installing a high flow cat. I found a screaming deal from a fellow MCF'er about four months ago on a ZZP with a high flow cat. I changed it out shortly thereafter and it took about three weeks for my code to come on (I'm a pessimist, I kind of expected it). Anyway, after driving around with a CEL for three months I finally ordered the non-foulers last week. Thankfully my O2 sensor was easy to get out of the downpipe since I anti-seized it when I replaced the downpipe four months ago.

The whole job was 20 minutes, not counting running to Lowe's to get a 1/2" drill bit. Which brings me to my one bit of confusion. I had always read you have to bore out the non-fouler, clearly I misinterpreted what that meant. I thought that meant I had to open up the 1/4" hole (on the very bottom) to about a 3/8". That's technically true, but you also have to bore out the entire ID to a 1/2" diameter. A new bit, a vice, and a bottle of water as coolant and that was only an extra five minutes.

Anyway, the CEL turned off last night on the way home from work, and I changed it Saturday, so it took me 12 drives (of varying length and speed) but it finally turned itself off.

This is what I used;