P0420...which cat is bad?

I recently bought this 2002 3rd gen with 102K miles. I have all the repair records that indicate that the front cat was replaced at 62K miles in 2015. I have determined that my O2 sensors are good and performing correctly. It appears I have a poor functioning Cat. The issue is which one ? Since the upstream B1S1 O2 sensor is ahead of the first Cat and the downstream B1S2 O2 sensor is behind the 2nd Cat. It also appears the 2nd Cat was replaced as well because it is not factory. Any help would be appreciated. I have read many threads on this Code but can determine which Cat is bad. Should I just replace both ?

 

Any help would be appreciated. I have read all the posts about P0420 code and none address the Cat itself. Should I replace one at a time ? Both and both o2 sensors ? That P0420 code is really none specific.

 

I am not that much, if any, in the know when it comes to emissions but I'll ask some probing questions. I take it that you have access to an OBD tool. Is P0420 code the only code that the OBD tool is displaying? Will you be able to use the tool and take real time readings for O2 sensors and anything else that the tool can show for the cats? I believe you should see a difference in readings between the two O2 sensors and if they're the same, then the converters are not doing their job. Typically what goes bad are the O2 sensors. Converters last the life of the vehicle, at least based on my experience.

 

Yes I do have an OBD tool and it only shows the P0420 code. I have reset it 3 times and each time after about 5 startups / 150 miles the CEL and VSCtrak off light come on. The Same Code pops up. I can see that the front o2 sensor voltage is fairly stable around 3.5V. The downstream o2 sensor fluctuates between .65V to a low of .35V. I though the same thing about Cats lasting a lifetime. It appears the rear Cat has been replaced because it is not original. I have a record of the front Cat being replaced. Thanks for any advise....

 

Doing a google search (https://www.fixdapp.com/blog/p0420-toyota) it says if there is a fluctuation between 0.1v to 0.9v then the cat has gone bad but then your fluctuations are tighter. Does the OBD state what the nominal voltage values are supposed to be for both the front and rear O2 sensors? Since both cats have previously been replaced, I don't understand why they would have gone bad again! Does the engine burn oil?

 

No doesn't burn oil. Only 102K actual verified miles. I will check the OBD and see if it states what the nominal voltage. The o2 sensors have been replaced in the last 50K miles as well. Do aftermarket cats create a problem ? The front cat appears like a Toyota replacement and the rear appears aftermarket.

 

I'd stick with sensors, pcv valve, look for exhaust leaks first. Also if any of those sensors are not Denso, change them. Cats can last a very very long time, I'm surprised they were already replaced. Alot of good threads on this at tacomaworld.com & www.t4r.org

If there is any chance of head gasket leak, coolant will burn up the cats.

https://www.tacomaworld.com/search/13137370/?q=P0420&t=post&o=date&c[title_only]=1

Also engine determines bank 1 or bank 2. Bank 1 is passenger side for V6

 

If cats have gone bad again (and I have a hard time believing this to be the case), then I surmise there is something wrong with the engine (ex: oil burning). What OBD tool are you using?

 

Driven the 4runner 1500 miles and no issues with oil burn or low oil. Also...oil is clean not frothy. Thanks for all your input. I will keep you posted.

 

I only used oil burning as an example. What I was trying to say is that if the catalytic converter has gone bad twice in 102K miles, then something is wrong in the engine and the failure of the cat is merely by-product of the primary issue. I'll go back to OBD and see what the tool says about the various operating parameters in the engine and whether they're within nominal or not at different engine loads.

I know I am not helping much here but some thoughts anyway.

Certainly keep us posted on what you learn.

One other after thought. On my 97 4Runner, the two O2 sensors are identical. They have different part numbers because of the lead length. Anyway assuming that this is also the case for your 4Runner, you can swap the O2 sensors and take another OBD reading. If the fluctuation in the rear O2 sensor which is now mounted in front, continues to persist, I'd gather that sensor is no good. However if the front one which is now mounted in the back starts to show a fluctuation, then the cat may be at fault.

 

There is no way you can know whether you need an O2 sensor or an A/F ratio sensor unless you *contact the dealer with your vin*.

On my car, the downstream one is an A/F sensor. Every parts store said I needed an O2 sensor. The dealer was the only one who could sell me the right one.

They look identical.

 

So I decided to run some Catacleaner and 44k through my system after I cleared the code. it improved my gas mileage and after 400 miles no codes have popped. If it does I plan to replace the rear Cat and rear o2 sensor. Then move on from there.

 

So the Cataclean with BG44K did not work. P0402 showed up after 300ish miles. Going to start to replace the rear o2 sensor and the rear Cat. Stay tuned.

 

They had a recall on my cat...,,,..Got it replaced for free.....,,,,Maybe check into it..

 

Here is what you can do, go to a junk yard and find a 3rd gen and snatch the O2 sensors out of there and try in yours. Cheaper than buying OEM for now. Just clean them really good with Sensor cleaner (not brake cleaner). I hate Yota added an additional cat on 2000+. My 99s only have one thank God and I replaced it with ease as the previous owner let it get gunked up and honeycombed (pictures coming soon).
Hope that helps a little bit.

 

So I have fixed the problem....I took apart the entire exhaust from the manifold to the muffler. I inspected the cats by shining a light and used mirrors to inspect the inside. They were clean as a whistle, open and in goo shape. So knowing that....I replaced the rear O2 sensor and reassembled the exhaust. Been 500Miles and no CEL or VSC/Trac off light. So...if you get the P0420...start with the rear O2 sensor. I found a Densco for $35 online. Thanks for all the help !!

 

So I have fixed the problem....I took the entire exhaust from the manifold to the muffler. I inspected the cats by shining a light and used mirrors to inspect the inside. They were clean as a whistle, open and in goo shape. So knowing that....I replaced the rear O2 sensor and reassembled the exhaust. Been 500Miles and no CEL or VSC/Trac off light. So...if you get the P0420...start with the rear O2 sensor. I found a Densco for $35 online. Thanks for all the help !!

 

Congratulations. It is always great to hear success stories.

 

Has the CEL came back? I had the P0420 on my 2002, changed both sensors with Denso ones and the CEL came back after 700+ miles. I’ve reset it a few times and it’s still coming back.

I plan on taking it to a shop to get the sensors read to hopefully determine if the cats are actually causing the CEL.