So I bought my 4R over the summer, maybe 4 months ago or so with 142k on the clock. When I got it, everything ran perfect. However about a month in my CEL came on, along with two traction control lights. O'reilly's told me p0420 cat converter/02 sensor code (i know, been covered many times. ive read through them all).

I deleted the code and it didnt come back until a long while later (probably 600+ miles). I checked and it was the same code, so I wiped it while looking around and gathering info. the next time it came on was even longer of a gap.. about 700 miles. this time I took it to a toyota dealership (free diagnostic) and left it there for 4 hours. they told me cat converter went out, not 02 sensor.

Here's where my question comes in..

I inquired as to their testing method, since an 02 sensor going out will trigger the same p0420 as a cat converter going out. I asked if he had the print out of the diagnostic report (i wanted to check voltages across the two 02 sensors). he responded by saying that their machine isnt "one of the hand held ones you find at autozone, its a full computer". I said I agree, however i just want to make sure its the cat (est stealership cost $1500+) and not a simple sensor malfunction, since the deficiency of one can cause the resulting code to falsely represent the deficiency of another. to that he just said "we plugged your truck into the huge computer and it specifically said cat converter".

I have no other way of testing the 02 sensor voltages to know for sure since i do not have a scanning tool capable of that, nor do i have any "hook ups" in terms of shops where i am going to school. I know the truck has a lot of miles on it, but i'm tempted to take the gamble and buy the $70 sensor and just try it.. ideas? any other way of testing this? feedback? cats can go out whenever, but the general consensus on this board seems to be the cats outliving their respected 02 sensors.

sorry for the rant. just wanting to make sure the dealership isnt trying to screw me by pulling the code and its most expensive fix, rather than investigating the issue fully.

I can tell you my truck acts much the same way, I almost always know that a long low speed drive will trip the check engine light for the same code. Anytime the engine is warm and the cat can cool off a significant amount it throws the code. I know its the converter, simply from the smog tests here showing the same thing. Cold truck wouldn't pass, warm truck barely passed. Next round it will need a new cat to pass.

as far as testing the o2 sensors go, They readings vary on the amount of oxygen in the exhaust. The pre cat o2s will bounce back and forth on a normal running engine from low to high voltage as the ecu constantly leans and richens out. If your engine is running really lean or way too rich you may see the sensor read more steady to one voltage with less bouncing back and forth. For the post cat o2 which you are more concerned with it will read a more steady voltage because the cat should clean up the exhaust. If the cat is bad the sensor will probably either bounce around or read rich. most o2 sensors read from .2 - .7 volts lower voltage means a leaner more oxygen in the exhaust and higher means less oxygen. So as far as testing the sensor if it is a little off it can be diffucult. Youd need to watch the readings and kinda determine that from experience or some cars you can swap them with another post and pre or from bank to bank and compare readings. I hope this helps you out.

agreed, Richard808. I understand what you are saying. im just having a hard time finding a device to do this. I asked the dealership for their print-out of the diagnostic, but he kind of ran around the question by saying that their "super big computer" spits out all you need.

if its the cat, so be it. i just want something that tells me for sure. otherwise ill take the gamble on the sensor. regular shops will just read p0420 and look it up as a cat deficiency code

I was in the same situation as you are. My P0420 would set every two months or so at first and over the course of a year it came back progressively more quickly, until it was just a few days until it returned. I had already done the front A/F a year or so previously so I took a gamble on a swap of the rear O2. I put in a Denso I got off Amazon and in two years I have seen two P0420's: both after extended low speed travel.

We had a guy on YT claiming to be a Toyota tech saying O2 sensors couldn't cause P0420's and it was always the cats. I wouldn't trust a dealer on this, unless you have some other indication the cats are bad--smell, noise, visible damage, performance, bad tailpipe test, etc. Take your chances and spend the $70. At least you'll have a new sensor to go with your new cats if it doesn't work. And if you have to pay for it, any diagnostic will cost more than that and you will still have an old sensor.

See this thread, we've been down this road before:p0420

thanks durk. i think you nailed my situation. i just have doubts on the dealership based on his answer. he wouldnt show me the report/voltages. he kind of answered like i have less automotive knowledge than i do by saying "we use a real computer, like this one here". i dont care how big it is; what does it do .

anyway, i only smell exhaust with all windows down, cold start, immediate reverse. otherwise nothing. noise isnt too bad, maybe a gasket needs replacing though, sometimes i hear a similar sound. performance has been a little lower when the code is on (i think its the ecu running a safer fuel map since it cant rely 100% on the 02 feedback, personally), but im still getting 18mpg (+,- 2mpg). valid point with diagnostic charges higher than the sensor. my only other option is to wait until the holidays when i will be back in my home city with contacts equipment to borrow to scan it (shop teachers, shop owners, mechanic friends, etc.)

looks like ill place the order for the sensor

Might as well try a new o2 sensor. If nothing else it will probably need replacing soon anyway. I hear they start to go bad/degraded performance after 50k miles anyway. My only experience with a bad cat was in my 1987 Ford Thunderbird when a pre-cat/cat melted down. The car wouldn't rev over 2-3k rpm because exhaust gas would back up and put pressure on the new fuel coming in.

P.S. since I have put my new O2 sensor in my 4runner(160kmiles) has been averaging 1-2 mpg better than it was. Where before if I was doing 75-80mph I would get 17-18, now I have yet to see under 19. (though to be fair I also cleaned the MAF).

I'm having the same problem as well. Same code. I reset it and it came back on after 90 miles. I reset it again to see how long until it comes back on. I can't get the rear O2 off and people keep telling me they are 100% sure its the cat then other people tell me they're 100% sure it's the rear O2. What do yall think about this guys solution to cleaning the cat by pouring a gallon of lacquer thinner into the gas tank at half full and running it for a while to clean the cat?

I saw that video when I had the code. I opted to replace the rear o2 sensor instead, which solved my problem. That said, I wasn't convinced lacquer thinner was the best substance for my engine, so I quickly discounted dudes experience and suggestion. If you decide to do it, please report back with results.

I like the idea that the rear O2 needs to be replaced over the cat but neither have been done to my knowledge in 100,000 miles. I just couldn't understand how so many mutual people could be so sure it was one or the other. Price isn't a huge issue until I have to replace everything (both O2's and cat) they are all around $100 for me but I'd obviously rather just do one.

That is a very interesting solution. I don't know much about lacquer thinner, but if this is legit, I am definitely going to try it. I sure hope the lacquer thinner works, because I welded the 2 cats together to seal the rusted away joint between them, so they're going to be a pain to remove

After a lot of reading I would advise AGAINST doing the lacquer thinner. They make cat cleaners such as catacleaner and other at your local auto parts store. I would shoot for these first for several reasons that I read.

I had the po420 code also. I reset the light and 3 or 4 days later, sure enough it's back on, same code. Last week i did a BG fuel injection service, reset the light and "knock on wood" no light thus far. I've put 300 miles on it this weekend alone on a atv/camping trip. I've performed this service on customer's cars with the 420 code with great success on most ocassions.

For the price and effort of installing a new rear sensor, why dont you forget trying to test the old one and just put a new one on and see what happens? Thats what i did and it has been several months now and so far no codes have come back. Cost me 62 bucks for a denso sensor and a few dollars for new nuts.

Exhaust leak can help throw the code too, as I recall.

Of course the dealership wants to sell you the most expensive thing.

No one can diagnose the thing over the internet. Pick a direction and try it out. You'll get out of the cornfield eventually.