I have a 2003 4Runner Sport with 180k miles on it and I've been having this p0430 code on for a while now. AllData shows me how to test the vechile and that it can either be a cat, o2 sensor, or an A/F Sensor. I want to know your guys experience with this code because the cat is expensive to replace... the A/F is not only expensive but it's a whole b**** to get out by the position it's at. I want to change out both the o2 Sensors but I just want to know that's the issue. Anyone have experience with this and can help on how to fix it?

First thing to look for are exhaust leaks. Even a small leak can throw this code.
There’s different tests you can use like running seafoam to check the smoke leaking or running a vacuum in reverse to blow air into the exhaust pipe.

Next is to check the temps of the exhaust pipes pre and post cat after a long drive. Amazon sells IR thermometers for $20. If it’s hotter pre cat then it’s restricted and needs to be replaced or you can try cleaning it.

If it’s not the cats , next would be to change the downstream O2 sensors. Change them both and only use either Denso or Toyota OEM sensors.

Upstream AF sensors shouldn’t be causing this code.

There’s a few long threads on here with lots of user experiences, search for P0420 and P0430.

Awesome sounds good... thank you for the advice. I will do this and just hope that it's not the cats... because money is tight right now lol

I had P0156 and P0157. It was a bad bank 2 sensor 2. Replaced with Denso unit and its been fine for over 200 miles now.

As a master technician with over a decade of experience, this is not necessarily good advice.

A clogged cat usually results in sulfur smells and hard starting. If you have neither, it's more likely the catalyst substrate has been used up.

Barring no exhaust leaks between the end of the cat and the downstream sensor, you very likely have a bad cat.

A bad secondary O2 sensor will throw its own code.

So, if you don't have an exhaust leak and additional codes for a downstream O2 sensor, you're very likely looking at a bad cat. Therefore, replacing your O2 sensors will only result in wasted time and money.

All this said, and over a decades worth of experience wrenching professionally, I'd place a gentleman's bet of $1 that you have a bad cat and I feel very good about placing this bet.

With a bad cat you have two options; replace the cat, or google spark plug antifoulers in conjunction with a bad cat and follow the steps at your own risk.

I speak from experience , I own the same car as the OP and had the same problem code p0420/p0430, twice in the last 2 years in fact. First time was cleared up by changing the downstream O2’s and second time was an exhaust leak a year later. Cats haven’t been touched and many on here have had similar experiences. There’s several long threads on this trouble code. And if I listened to mechanics like you I’d be several grand in the hole.
And you’re encouraging expensive cat replacement without seeing the truck?
It’s no wonder most on here do their own work.
It may very well be the cat in the end but how about we try to save the guy some money first?

I agree, and in my experience CATs only go bad if they’ve had problems with extra unburned fuel being ignited inside by heat from the CAT. This usually caused by failed coils, or other ignition/fuel related problems. CATs don’t typically just wear out. They either get plugged, or internally damaged by raw fuel as described. OP, exhaust leaks are super common on these trucks, and cause a lot of these problems as others have pointed out. That being said, if money is an issue AND you think it’s actually the a CAT that’s bad, you can certainly try the anti-foulers on the rear sensors. It usually takes 2 per side, but it will work in most cases. I would exhaust (no pun intended, lol) all of the diagnostic options first though, to try to correct it if possible. Good luck, and please update once you have it solved.

If you listened to me you'd have a vehicle that was fixed properly the first time.

Please re-read my post to answer your first question. I didn't recommend anything. I did, however, provide factually accurate statements, my hunch as to what's causing the problem, and information on how to fix it.

Do you have any empirical data to back up your claim that replacing downstream O2 sensors is the fix for a catalyst inefficiency code?
Remember, correlation does not equal causation.

I know this may be hard to understand, but your p0420 and p0430 were not fixed by replacing downstream O2s. The codes simply never set after you cleared them when you replaced the sensors. I do, however, have no doubt that your exhaust leak could have caused catalyst inefficiency codes as this is very common.

Because of the sensitive nature of catalyst inefficiency code logic (dictated by the gov, and different in California), every dealership I've worked for has a policy of clear and wait for re-code on P0420 and P0430. Interestingly enough, I'd say less than 1/4 of vehicles would re-code and require additional diagnostics.

As a side note, 99% of career technicians I've worked with are looking out for the best interest of the customer. I, in no way shape or form am trying to dupe people into pointlessly spending money. Most of us take pride in accurate first time diagnosis and repair (a common metric in the industry). Stating I'd cost you "several grand" and am part of the reason people work on their own vehicles, shows your misunderstanding for the industry, my post, and myself personally.

I could very easily make the argument that listening to somebody uneducated in the field of automotive repair that's recommending replacing downstream O2 sensors for a catalyst inefficiency code is the easiest way to waste "several grand", require multiple repairs, and is reason to have a professional look at it.


I'm glad that you're both aligned on your logical fallacies, but allow me to provide some actual facts as opposed to the tribal knowledge being passed around. Catalysts do, in fact, go bad over time. The catalyst substrate is made of platinum, palladium, and rhodium. Heat cycling of the substrate causes it to break down over time and the microscopic parts or platinum, palladium, and rhodium get shot out of your exhaust. So yes, prolonged loss of substrate material does mean the catalyst becomes less efficient over time, which obviously, can result in the codes being thrown.

I didn’t say they NEVER go bad, just that they typically last well over a 100,000 miles, barring any engine issues causing early demise such as the examples I mentioned. I actually had one go bad at 60k miles due to a coil related misfire on another vehicle. My intent was to not have the OP just throw an expensive CAT replacement on without doing due diligence, as it could very well be something much simpler, and cheaper to repair.

My p0420 cleared and stayed off after I replaced my exhaust donut and O2 sensor.
I was having to clear it every day. I assume it was one or the other causing the problem, but had purchased the sensor already.
Check the exhaust donut first if you want a cheap fix. I couldn't even hear a leak, but it was smashed so thin that there wasn't tension on the springy joint.

I had the same issue and I tried CAT clean and it helped out for awhile. I also replaced all the Co2 sensors. 6 months later, the PO420 code came back and it wouldn't go away. So I ended up buying a new CAT and I haven't had any issues since and that was 2 years ago. Parts and labor was $1000.00 at a local shop in Nor Cal.

If you do a search on T4R, you'll find that this issue seems to be common on the 4th Gen V6. Good luck.

You're right, you didn't say "never", you said "only." Which in context of the conversation, means the exact same thing.

I believe this proves my point.

I'll never understand why some on this forum believe that catalysts don't fail and resort to replacing known good parts as a solution. The stigma and false information about P0420 and P0430 codes around here is rampant.

Same issue. Tried doing the cheap fixes first. Gas cap, then couple hundred miles later, would appear, then did the O2 sensors one at a time. Thought it was fixed but code would show up again. Finally did the cats Worked. haha

After a catastrophic accident when I accidentally drove into the ocean and then huge backfiring sounds came from the catalytic converters, the 0420 code was set. This was obviously caused by explosions blowing part of the grid apart when I got saltwater in the engine. Rather than fork out thousands of dollars for a new driver side cat or risk getting caught with sensor spacers (literally everyone gets caught during the smog check with defoulers in Northern CA nowadays) I figured out how to make a completely blown or removed cat read as a perfectly operating cat and never set a check engine light ever again. Note that there are several "o2 simulators" which are little more than a feeble attempt at delaying the upstream cat signal by 90 degrees. What you really want is a smoothed output of the correct voltage which still responds to the upstream o2 sensor inputs in both the correct time base and reports the correct voltage.

So here is what to do: solder a 6.7K resistor in series with a 4.6uF low esr capacitor onto the blue sensor return wire and slide the wire heat shield back over your circuit so that it looks like nothing at all has been done to the sensor or the wires.

There are several posts on the interwebs showing an obviously incorrect circuit placing a capacitor in parallel with the sensor element as an RC filter...This will definitely not work on a 4th gen as the computer expects constant current and no change in VI relationship across the sensor. The computer detects your wierd RC filter circuit and sets a code stating the computer is bad and now you have created an altogether different problem. Which is lame. What you want is a circuit that acts as an adder, meaning you simply add a negative voltage to whatever voltage the sensor is reading. Unfortunately this will only work with completely blown or removed cats and will throw another code if you use a modified sensor with a good cat.

I am going to smog my truck tomorrow and see if it passes. The rear cat is still installed so hopefully that cat has enough material left to filter out the pollutants. Anyone want to place bets that I can pass smog with an exploded pre-cat?

Well....
did it end up passing the smog test?