First of all, the more I thought about it, the more I thought that starting a new thread rather than adding on to my previous one was the right way to go, I may be wrong. Next, I'm not exactly sure how to do a search for all of the symptoms and all of the attempts I've made, so I'll list them below. 2000 OBW, 2.5L auto tranny. 170k miles I've been dealing with this since I bought the car at 130k miles. It had bad headgaskets but was otherwise well cared for. I replaced the headgaskets and then the cats with aftermarket ones as well as a new Subaru front O2 sensor. Because of another problem, I have no idea how long the CEL was off with these cats. Several thousand miles later I figured out that the 420 code was back on and refused to allow my car to be touched by the previous guy who did my cats despite the cost. I then replaced the rear O2 sensor, the front O2 sensor (both Subaru) as well as the cats, this time with Bosal. At this time I also replaced the plugs and plug wires with OEM. A few weeks after all that, and a few hours after passing my emissions test, the CEL returned. I then used the antifouler trick and the CEL remaind off for a little over 2 months. Until last night. CEL came back on. I thought I felt a little bit of a roughness when gently pressing the accelerator from a stop. I have also noticed a 1 to 2 mpg decrease in my mileage. What's next in the line to check that I can check without a high dollar scan tool? What more info can I provide to help those with much more knowledge than I have to make a best guess on the next step in this process?

I had an issue with this code on another car and it ended up just being a glitch with the Engine Controller's code. The dealer flashed in new software and it never came back. Might want to call the dealer and ask him if this is the case.

Otherwise, if you think the system is running fine buy the O2 sensor simulators. I run a cat free exhaust in a street legal rally car. So I don't get nipped at the emissions place I plugged the O2 connectors into these babies.

O2 Simulator

 

Code goes away... I get better gas mileage without the cats.