I've had p0420 before just a couple months ago, and I read a lot of threads saying how it could be caused by a number of things besides the catalytic converter, but at that time I was pretty sure it was the cat because my dad had cheap 48 state legal ones installed before I got it. My 4runner is a 2002 so it has 2 cats, but when it came down to buying OEM catalytic converters I decided that I might be able to get away with just replacing 1, the rear one. Now this could be the cause of the problem all together and don't have any problem denying it, but I want to be absolutely certain before I drop the cash on another new cat. The rear O2 sensor is brand new as well so it's really making me think I just need to get the new cat.
p0420 with new catalytic converter and O2 sensor
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Malcolm99
Member
Try other things like front A/F sensor, clean/check MAF, IAC, TPS, coolant temp sensor, check/change plugs wires, compression test while at it, eliminate any combustion related items before changing the cat(s) as the OEM cat is probably the best option available and usually last a long time. If the front A/F or O2 sensor isn't working perfectly it could run rich and you can get p0420 as the cat is seeing to much fuel, as can any of the fuel trim related sensors.
19963.4lsr5
New member
How often do you have to top off the antifreeze overflow tank?
Antifreeze induction is a common cause for the 420 code.
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Antifreeze induction is a common cause for the 420 code.
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I actually cleaned the MAF sensor last month after reading that it was a good thing to check up on. Other than everything else you mentioned, I have a bluetooth OBDII scanner so I was able to look at some graphs of the O2 sensor and the A/F sensor, and after googling how they should behave, it looks like the voltage from the A/F sensor doesn't seem to line up with the O2 sensor at all, most of the time at least, sometimes it does line up but the voltage only changes by something like 0.1, and other times the O2 sensor voltage will go up or down from 0.1 volts to 0.8 and the A/F sensor voltage doesn't change. This was just doing a quick drive around my neighborhood after I let the engine warm up though. So should I do more graphing or does this mean the A/F sensor is dead?
so after you clear the code, how long does it take to come back on.? My 2000 when it had a bad o2 sensor, I could clear the code and it would be a while b4 coming back on...like many miles. Now this one, my 2001, I can clear the code and it will come back on within a few miles....like under 5. I have a new Denso one on order. I have had issues with non denso
I just did plugs, wires, maf and throttle body cleaning, a new coil, compression test, leak down test and test the cooling system ( and I have no fluid loss) so I know it either has to be the o2, a cat or the AF, and the o2 is cheapest placce to start, and has been the guilty party in the last toyota's ive owned.
I do have to add it could be a ad cat, as I recently bought this as a fixer upper, it was running on 4 cylinders due to a bad coil and plugs, so I dont know how long it was dumping raw fuel into the cat.
I just did plugs, wires, maf and throttle body cleaning, a new coil, compression test, leak down test and test the cooling system ( and I have no fluid loss) so I know it either has to be the o2, a cat or the AF, and the o2 is cheapest placce to start, and has been the guilty party in the last toyota's ive owned.
I do have to add it could be a ad cat, as I recently bought this as a fixer upper, it was running on 4 cylinders due to a bad coil and plugs, so I dont know how long it was dumping raw fuel into the cat.
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19963.4lsr5
New member
The rear O2 sensor should never be in phase with the front sensor. The oscillations should be lower in amplitude (voltage) and should be a slower ripple.
When the pcm sees the same reading front and rear it assumes the cat is not catting.
Do you have any exhaust leaks? That includes the O2 sensor gasket base rusting . This will also cause a 420 code.
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When the pcm sees the same reading front and rear it assumes the cat is not catting.
Do you have any exhaust leaks? That includes the O2 sensor gasket base rusting . This will also cause a 420 code.
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I actually did have some exhaust leaks recently giving me p0136, I will check again today and make sure there's no more. So at this point I guess I can assume the A/F sensor is fine, and I hope the oscillations the O2 sensor reads is just showing that I still have the old cat and not that the new one is bad.
19963.4lsr5
New member
Sadly you need to punch all the buttons in a row. That includes replacing the other cat to totally eliminate it.
My 420 was from antifreeze. Your not topping off your expansion tank I hope. I look at my level every time I fill up and check the oil level.
My friends taco was a bad rear sensor and he installed the new one without the gasket. After he put the gasket on his 420 went away on his next drive.
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It looks like there might be something up with the antifreeze after all. When I looked at the tank yesterday it was after driving so the engine was hot and the level looked okay. Looked just now, engine stone cold, it's at the low line. This could mean any number of things though, so first I'm going to check for leaks. I've never seen anything dripping from the front before but I'll have a look. I've never monitored the antifreeze and oil level as closely as you say you do before, but it must have taken at least a few months before the antifreeze has gotten to this level. Oil level looks good though. I'm still going to rule out an exhaust leak.
19963.4lsr5
New member
Top it off to the full Line cold and watch it. Slow head gasket leaks show up as minute loss and that’s enough to effect the cat. It cools it and also Coates the ceramic.
Most slow leaks will be hard to find because because it will evaporate from the hot engine.
Good place to start looking for leaks is the heater control valve and it’s associated hoses. If you have the heater under the passenger seat those pipes rust out unless you have the retrofit stainless lines.
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Most slow leaks will be hard to find because because it will evaporate from the hot engine.
Good place to start looking for leaks is the heater control valve and it’s associated hoses. If you have the heater under the passenger seat those pipes rust out unless you have the retrofit stainless lines.
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Topped it up and started the engine to look for drips in the places you said, couldn't find any. No drips from under the car either. I'm sure if I had one of those pressure tester things I could do a better job finding any leaks. At this point I might go ahead and take it in to a dealership and have them check everything out. Not only am I dealing with the p0420 but I also have some evap leaks, p0440 and 442 have both come up. I'm not well equipped enough to move forward with anything else but thank you for all the advice anyway! Hopefully I can get all this sorted out.
19963.4lsr5
New member
A lot of times those EVAP codes are from the two rubber gaskets on the tank leaking from old rubber or rust on the bare metal at the gasket surfaces.
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dealers can be expensive and maybe find a good independent near you, or ask on the forum for recomendations for in your city. I have seen people ask on forums in new threads for recomendations for shop in "x" city.
For fun I looked at the texas club, it has not seen much activity. I wonder if these clubs would get more use if they were easy to find. My state gets almost no action.
https://www.toyota-4runner.org/texas-and-oklahoma/
Also , someplace I saw a list of T4R owners listing each state and peole that can help. Maybe find that thread and see if someone is in your state/town and able to help or point you to someone that can. Maybe it was in the "frequently asked questions" thread
Hi I have the exact same situation right now where my upstream A/F Sensors is holding at around 0.6V and my downstream O2s will either fluctuate or hold between a slightly higher voltage. Did you end up figuring out the issue?
Sorry for the really late reply but I ended up replacing the entire exhaust and it was able to finally fix the code. Unfortunately not a quick and cheap fix! I think the main reason was because only 1 catalytic converter was new and there was also leaks along the entire exhaust. So I gave in and spent the money on an all new exhaust, from the catalytic converters to the tail pipe, and it finally got rid of that cursed code. It's been several months now and everything is still going strong.
Endlessblockades
Senior Member
My P0420 was simply a too-loose exhaust pipe coupler on a section of the cat-back exhaust system I bolted on. Just mentioning it because this code can be triggered by leak issues past the sensor and cat(s) and can be a free fix.
JoeT4R
Senior Member
Would you be able to post up a pic for reference of where your leak was coming from? I've had the P0420 for the past 10 years and I've just let it ride.
Not sure if I need new cats or if I have a leak. Never put the time/effort into really trying to get it squared away.
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Endlessblockades
Senior Member
My situation might be a bit different because the kit I am running has multiple bolt-together segments and resonator. That's where my leak(s) were. Besides the main flange behind cat 2 on a CA rig, I'm not sure if there are any other junctions beyond the cats in the stock pipe. Now that I think about it, I had to cut the stock pipe with a small hand-held hacksaw to get it out because I couldn't wiggle it out sitting on the ground. Have you looked for small rust holes along the system? My leak(s) were downstream of the 2nd O2 sensor. I'd look at any other possibility before buying a cat.
