fullmann

my cel has been on for months, p0420 every time.

I cleaned the maf, as well as new plugs, wires, fuel filter, all the stuff short of new o2 sensors, and new cats. I did do the deckplate and isr mod, but cel was on before and after. zero running issues, only symptom is the light. in fact, my gas mileage has been incredibly good, 21+mpg.

today, I fired up the truck, the light cycled, and stayed off. I drove it a couple times with the same result.

I wouldnt think it could fix itself, but I cant really argue.

anybody else have this happen? or is this what happens right before the truck self destructs??

00 sport, 5-speed, 99k miles

86toysr5

mines on in my 97 i changed air filter and clean maf but when i clear mine it then it comes on it only does it on the interstate lol

MTL_4runner

I think time will tell. It is possible you have a borderline cat or that you had some other issue causing your cat to work too hard and the tune up somehow fixed that issue. I'll be curious if the CEL is still gone a few months from now. If that is indeed the case it would be pretty uncommon.

jdsdj98

PO420 = Catalyst System Below Efficiency Threshold.

I started having the exact same problem with my truck after changing both O2 sensors out about 10 months ago. By definition, I interpret it to mean I need a new cat, although the truck "Fast Passed" its emissions test last July. That just confirms to me that emissions is a scam, anyway.

You're in Colorado, too. I've noticed that mine comes on much more often in 80+ degree weather. I cleared it back in October (roughly), and didn't see it all winter until it finally came back on last night. I cleared it this morning and don't expect to see it again until our next warm spell. In the summer heat, I can expect to see it 36-48 hours after each time I clear it. It's a pain in the butt code, for sure. I'll eventually get around to changing the cat, but haven't made it a high priority.

Have you noticed the same thing, i.e. that it comes on more frequently in warm weather? I wouldn't think it could fix itself, but our weather has been cooler the past 2 weeks, which could explain why you haven't seen it as often. Give it some more time, clear it as quickly as possible each time it comes on, and take note of the weather. I can't explain why that's the case, but that's the determination I've made. Maybe someone else here can explain why this is.

fullmann

came back last night, I guess I knew it was only a matter of time.

I just cant bring myself to pay $2k+ for new cats from the dealer, being that I have a cali emmissions truck and anything but factory cats will throw the code again.

oh well, I guess i'll wait til I have running issues then go from there.

MTL_4runner

I thought so.
Run the URD O2 simulator if you need to.