I know there have been countless threads on this error code, but since I got my 4runner down to Florida it has been popping 420 codes. I have had it cleared a few times, no big deal.

Here's the strange part though. The other day I was playing around at one of the state parks, going a bit fast on the dirt roads with my 4wd on, and the strangest thing happened. The check engine lights turned off. And actually stayed off for a couple days.

Sadly they are back on now though.

My question is: What the hell? A P0420 code probably shouldn't clear itself like having the gas cap loose. I thought it stayed up until cleared. So why did it clear itself?

Also what the heck should I do about it? Should I try finding a new sensor and replacing it, or should I just go do a cat back?

Have you tried just taking the O2 sensors out and taking a look if they're still good? I know its a bit of a long shot since if that's the case it'd throw you a constant code, but who knows maybe it'd work. haha

I've had the code for over a year. I replaced both o2's and it went away for 5mounths and then it came back--I had it reset a few times and it would stay off for a few tanks of gas and it did shut off by itself at least twice. Made it through inspection last year by unplugging the battery and it came on a week later--Went in for inspection this past week and expected I'd have to get a new cat but the mechanic found a small exhaust leak near the cat. He welded it and reset the code. Lights been off for almost a week. Fingers crossed. Do you have a rotten egg smell or a cat that is rattling?

0420 can come and go if you're CATs are right on the line of efficient enough. It might just mean you are on the edge.

youtube how to clean your CAT, that might work for you if your CAT is mechanically ok, just dirty.

I haven't noticed any smell coming from it. Even my passengers haven't said anything about it. Pretty sure the same can be said about the rattling sound, I haven't really tried listening for it.

Hey, good idea. Never thought that just cleaning it would work.
I'll have to try that before I end up blowing tons of cash on new cats. Thanks!

Here's my thread on how I fixed the code. Replacing the rear O2 sensor did the trick.
Help fixing P0420

Never heard of "cleaning" a cat...

I only know as much as I've experienced and the two times I've replaced cats on a car was when there was a smell of rotten eggs or a rattle and with the rattle--sometimes chunks of the innards would clog up the pipe and cause drastic power loss. Hopefully my light stays off. Good luck--I hate having a CEL every time I'm in my Runner. I thought about cat cleaner but I never had the urge to spend $30 on a magic elixir--But if you go that way I'd thoroughly research the product.

I've had that code for years now. It didn't seem to effect my mileage, so I've just left it on. :|

Well, it's been a week and the light came on--on my way to work. Guess I'm going to need a new cat.

I know this is an older thread, but i didn't see the need to start yet another p0420 thread.

Anyways here is my deal. I have a brand new Catalytic converter, not a weld in one but the whole unit is new. Magnaflow from Oreilleys. It has been fine since I replaced it in March. 3 weeks ago I got the p0420 code and it says bank one so I assumed the bank one o2 sensor was bad and replaced it with a new Denso, code went away till yesterday. now its the same thing. The rear o2 sensor is probably 15 years old by the looks of it. I also had a misfire c3 but found the plug loose and tightened it(maybe coil is going bad?) Thoughts?

Things I have replaced/repaired.
New cat and new seals
New bank one o2 sensor
new vacuum lines
new Toyota gas cap
sealed charcoal can seam with JB weld for good measure.

You should have started with the rear O2. You may have trashed a perfectly good factory cat. I can't imagine changing the cat without trying the rear O2 first. Change it now. They will often throw the P0420 before any other code because they get 'lazy' with age, before they burn out and throw a code.

By the way, both sensors are Bank 1. Front is Bank 1 Sensor 1 and rear is Bank 1 Sensor 2. You only have Bank 2 exhaust sensors when you have dual exhausts which our trucks don't.

think I could have ruined a cat when the code showed up yesterday?

Needs converter