My P0420 code started this past February and it has been driving me crazy. I started with replacing the pcv, code still there, then I used the Seafoam method. That cleared the code for a few weeks just enough to help me pass my inspection in August. After that the code has been coming on persistently. I don't have drive-ability issue, the car runs like a champ, no lost of gas mileage whatsoever when the code is on.
So this past 3 weeks I have been dedicated my time to chasing this problem. I cleaned the EGR, my throttle body, flushed the radiator fluid, check the thermostat..no go. Replaced the spark plugs, new spark plugs wire. Tried the anti-fouler, first using 1 then using 2, that made it worst, throw me 3 more codes P2096 and P2098, and P0137. So I took the anti fouler off. Replaced the front sensor with Subaru/Denso sensor..no go, replaced the rear sensors with Subaru/Denso sensors, no go. I called the dealer, they wanted $2600 to replace the cat, not going to spend that much money on an 04 with 146K miles. So before I replace the cat converter which we all know is expensive even if I do it myself, I threw a hail mary and tried the diode hack. Viola!!!
It has been 3 days now with 6 drive cycles and no check engine light, all system back to ready when I checked it this morning with my code reader. Finger crossed. Here is the link to the hack
2000 Subaru Outback P0420 Electrical Hack Dan's Toolbox
Sorry for the long post and I hope this will help others that are having the same problem as me.