Code P0420

I know this thread has been dead awhile, but it helped me out, so maybe this will help someone else out. I have a 2006 4.0 V6 4x4 that started throwing the 4020 code. I would clear and it would come back about every 500-800 miles. The last time i got a p4030 as well, which is the same thing, other side. I watched the video and read the discussion about how it must be a bad cat etc. In the end, I figured IF the cat was bad, it was caused by too rich/lean fuel mixture pouring in, so I should at least change the upstream o2 (fuel/air ratio) sensors first since if I didn't, and I replaced the cat, it would just get screwed up again eventually. On a totally side note, the car idled a little rough, and when I shut the key off, it stuttered for about .5 seconds before shutting off. Also, in the morning when I would start it the first time, I could really smell gas. Now I didn't think these things were related at all. I checked my OBD11 reader just to make sure I wasn't getting any misfires or any other codes. Swapped the two upstream sensors out for new Denso ones and guess what. The rough idle went away. The car shuts off immediately and the gas smell is gone. So worth it just for that. Instead of clearing the check engine light myself, I decided to drive it awile and see if it would clear on its own, something it never did in the past. Light went off after about 50 miles and hasn't come back on since. So at least in my case, the o2 sensor replacement was the fix for this code. I went ahead and put new downstream sensors in as well the other day because their a lot cheaper than the upstream ones and probably about ready to go also (156k miles) Does not appear I will need a new cat. Hope this helps.

 

I just got this code this afternoon. I drove about 80 miles on this code. My truck didn't run any different. I'm going to inspect the exhaust and sensors tomorrow in the daylight. Hopefully it is something simple.
Thank you to everyone for all this crowd source of information.

 

Not sure if this will go to help you, but I ended up with the same code a couple months ago and was worried I would need to replace my Cat, but all that was needed was to replace the downstream O2 sensor. Could be super simple. Best of luck! Code turned off after disconnecting battery for a minute too to reset.

 

Resetting the code by disconnecting the battery just resets the emissions cycle with the ECU. Engine light turns off because you reset the emissions monitors and they have to start over. Takes a couple hundred - several hundred miles or so for the cycle to complete again and once it does, that code will be back

When an O2 sensor is failing they have specific codes. I would bet money that p0420 code will come back soon.

Catalyst inefficiency means one of two things usually. Cats are bad or you have an exhaust leak before the O2 sensor that makes it think the cats are bad

 

So I didn’t see any obvious leaks. The sensors didn’t look beat up. I didn’t pull them out. I drove around town for 20 minutes today. The CEL cleared by itself. I’m hoping it was a fluke.

 

That's exactly how mine was for about a year until it just stayed on constantly. Cats are going out

 

Did you try any fuel additives to see if it prolonged your cat life?

 

Cataclean seems to work well in a pinch if you need to pass smog. My aftermarket cats are about at the end of their service life and last year I needed to pass smog ... wasn't getting a p0420 code but my emissions/catalyst monitors weren't setting which is basically the same thing as far as smog is concerned. So I dumped a bottle of cataclean in the tank, drove around some dirt roads in 4lo for a bit and boom...my emissions monitors set to green. Passed smog the next day.

I don't know if it will work when the ECU is tossing a p0420 code but worth a shot

 

yeah. If it’s going out, I guess it couldn’t hurt it.

 

I had a similar issue with P0446 that ended up being a failed VSV that’s attached to the charcoal canister.

But there’s a trick that can work to passing smog even when you haven’t fixed the cause of the CEL. There’s a smog rule that says older vehicles can have one or two monitors in the “not ready” state so the trick is to clear the CEL and then get all your monitors except for the one that trips to CEL to be in the “ready” state and then you have the gas tank completely filled so it stops checking the monitors. (The EVAP cycle test only runs when tank is between 1/4 and 3/4 full).

Follow these steps until you get just enough monitors in “ready” state then stop and fill up the tank.

https://m.smogtips.com/OnlineForumQ...ion=How-To-Complete-Toyota-Tacoma-Drive-Cycle

 

This is from the FSM on how to diagnose this code.

 

Could you take a photo of page EG-272?
Thanks for posting these

 

Maybe so, but all I did was replace the downstream sensor and disconnect the battery and mine has not returned after having driven for 5k+ miles so far. Could be a fluke but seemed to work on my end. And for Dominick, I tried a bottle of cataclean and didn't end up with positive results, but again seems like every situation has different solutions.

 

Apologize for the late response..