I have been dealing with a P0420 code (Catalyst System Efficiency Below Threshold (Bank 1)) for the past two years, having episodic success with getting it to stay off for a while. Here is the deal... Back in 2007 I got the aging, stock muffler replaced with a cat-back system and a new high-flow cat. Six months or so after this change I got a P0420 light. No driveability issues, just the light. I disconnected/reconnected the battery and put some BG44K in the tank and the light stayed off for ~1000 miles of combined city and highway driving before coming back on. I then replaced the gasket on the rear O2 sensor and tried to clean it. Again, the light stayed off for ~1000 miles. With the light on for the third time, I replaced the rear 02 sensor and again, the light stayed off for ~1000 miles of combined city/highway driving and is now back on for the fourth time.

The truck actually passed emissions a couple of months back, and again, I don't see any MPG or driveability issues, and I don't hear or see any exhaust leaks. Basically, what I am thinking at this point is that I need a new CA-compliant cat to better match the stock cat's performance. Unfortunately I was an idiot and let the muffler shop who did the work keep the stock cat. If any one has any ideas on how to proceed, I am all ears!

replace that aftermarket cat with an OEM cat.

I'd say you're on the right track- put in a better cat. The OEM one is big, and has two sections of catalyst, basically two cats in one.

And don't fall for the "high flow cat" sales pitch. They don't even make the old "low flow" style any more. They are all high flow.

If you are in CA, new laws require you to use a CA approved OBDII certified cat, and the smog tech will check the numbers stamped on it (if he checks and notices its not OEM).

Yeah, the trouble with the OEM cats is that they run more than a grand, from what I have found. I could try my luck with another aftermarket "high flow", but... I am definitely kicking myself for not keeping the old cat, but at the time I had no idea how pricey the OEM ones were and I did not anticipate needing to go back to it. Does anyone have any opinions on the O2 simulator from URD? Seems like if I am genuinely passing emissions and not having any performance issues, this would be a cost effective way to deal with the CEL.

You might be able to pick up a used one from someone parting out their T100... <img src="/forums/images/graemlins/wink.gif" alt="" />

OK, so maybe an OEM-type replacement won't be so pricey...

Why not just ignore the code, and clear it prior to an emission test?

during an OBD2 emissions test the readiness monitors must pass. you have to go thru a complete drive cycle showing all monitored systems are fault free. it is designed this was to prevent ppl from clearing the codes around the corner and going in for the test.

Yeah, +1 on what vr4 said. Also, I really don't like the idea of having the CEL on all of the time because then I don't know if other conditions arise. Granted, its been on more than off for the past two years because of this issue, but if I can turn it off for ~$100 it will be worth it...

I understand that. This code isn't a chronic constantly on instantly retrigger code. It generally takes some time to re-establish and re-trip the check engine light. So what I said will work.

But since the OP doesn't like running around with the check engine light on, he's got to replace his cheap cat with a better cat.

Myself, I let the CEL stay on and generally ignore it. Come inspection time I clear it, cycle the system a few times with the engine warm, and go pass the OBD-II inspection.