Got a 1998 with 273K miles. Very well maintained for the last 15 years. Only use Top Tier gasoline. About every 3 tanks of gas I get a P0420. I clear the code and everything is fine for a while. I know the code refers to a catalytic efficiency issue. The state I live in does not do emission testing. 4Runner runs perfectly. Is this a sure sign to replace the converter? Or would you try an O2 sensor first?

Could also be an exhaust leak, however small it is, the ECU can notice there is not enough passing thru the cat and knows something does not add up.

I wouldn't worry about it unless it bothers you.

If you never replaced you’re O2 sensors that’s a first step. The rear sensor controls that code.

Exhaust leaks will cause it also.


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If you're throwing it that frequently I would check your rear O2. Might be dirty or on its way out. And it's the cheapest fix of the three
If that doesn't help you can try to blast the cat with a can or two of B12 through one of the sensor ports to wash it clean. Some run lacquer thinner through the tank with the idea being the cat will run quite a bit hotter than it normally does, burning off all the crap on it in the process. I've done it before myself, not sure how well it worked but it was pretty fun! Like poor mans boost
Not an uncommon code at all, many of us throw it once in a blue moon. I get it maybe twice a year

It could be anything related to air fuel mixture, including the front O2 and MAF, you would want to view the fuel trims to exclude these possibilities. If all else fails the OEM Cat is still available, whereas the aftermarket Cats won't last very long.

Is your exhaust system original? If so, I would check for small exhaust leaks first like Jgue467 suggested. I have had P0420s twice over 3 years and both times it was related to small leaks I had created for myself by swapping around parts on the MF Overland cat-back kit. Each time it was solved by tightening the connector bands on the modular system. These leaks were *beyond* the 2nd sensor, of course, so keep that in mind. Hopefully you can save some money rather than throwing parts at a pretty insignificant issue. If you are not needing smog, it's not a big deal. The stock exhaust may be rusting out from the inside. They don't last forever no matter how well maintained.

The rear O2 sensor is relatively cheap, less than $60. I would replace it.

Most common leaks are at the joints or at the O2 sensors themselves. They can be hard to spot so look for soot - exhaust gases making their way out of cracks.

I'm with the other guys, try replacing your O2 sensors. I would replace both. I would also not lose any sleep from the P0420 code. I've been getting that intermittently pretty much for the last 10 years. I mostly get it during road trips when I'm changing elevation. You're lucky you don't have to deal with the smog emissions like we have to deal with them in Commiefornia.

Adjacent P0420 question...sorry to hijack the post but I dont have enough responses accumulated to start a fresh one.

99 4wd SR5 with 210,000 miles. I got the P0420 about 3 hours into a drive home from the mountains. I called a local shop and they said 'almost certainly bad cat...' Have an appointment next week but want to cover my bases as much as possible before walking in and dropping several thousand bucks.

I ran some tests with OBDII and infrared temp gun. O2B1S1 (guessing upstream) reads steady .650-.655. O2B1S2 (downstream) fluxuates between .075 - .790

With vehicle running at normal temperature the infrared thermometer shows 291 before first cat and 253 after. 230 before second cat and 190 after.

Everything I've read and see suggests cats are bad. Any reason to think it could still be something less expensive? I read somewhere that the code reader might be switching values, as a healthy system would have fluctuating voltage upstream and steady downstream, but the temperature readings dont lie...

I've also read that the cats are supposed to last the life of the vehicle, so if it is the case that they went bad, I would like to figure out why. The vehicle has had misfiring issues since at least the previous owner, we've both replaced wires, plugs and fuel injectors, and wondering if the cats got fried from being subjected to misfiring at different intervals. Thanks for any insights.

Edit: sorry if it wasnt clear but its a CA vehicle

Did you see the post where the one member ran cat Cleaner and somehow his maf sensor was bad. It's a few posts down.

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My P0420 was solved with a new oxygen sensor and new gaskets.

I did read that post about the bad MAF in the midst of my forum hunting. Apologies if i wasnt more clear, my question was this - is the cooler temperature reading post-cat a sure fire sign of cats going bad? Youtube would suggest yes... but wanted to check in with those more experienced than myself.

After a longer drive 20 to 30 minutes if the front of your converter is hotter temp then the rear. You have to have a restriction. So if you have a single converter. It's most likely done. If you have a 2 cat system and the front cat is doing this, the rear one maybe restricting flow causing the cooler temperature. The front cat temp should never be cooler then the rear. [Except for start up]

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Hey Brillo, thanks for mentioning that detail about driving to get an accurate temperature reading. I'm guessing that starting up and idling up to operating temperature doesn't do the same thing. I'll take it for a spin, retake the temperature and report back.

I've had the P0420 code ever since I got my 4R. It comes and goes and I usually don't bother doing anything about it anymore. I replaced the O2 sensor and the other typical things, but it will usually pop up on long, slow drives at high outside temps. About 6 months ago (right before smog) I drove for about 2 hours on the freeway with O/D off at a high RPM to heat up/clean my cats. Worked like a charm and haven't seen the code since (although I know it's not fixed). But it's an option I suppose.