P0420 CEL Code Downstream 02 Sensor?

I have been getting a P0420 CEL code for a long time now, close to a year if not a little longer. I have read a lot of different things that it could be exhaust leak, cat converter, 02 sensors etc... So I am trying to change to downstream 02 sensor first. I believe I have located it but the wiring goes up into the cab it looks like so I am not sure how I unplug it and re-plug in the new one, or am I not finding the right sensor maybe. I don't want to have to tear the floor of the passenger side out just to unplug the 02 sensor, has anyone changed their downstream 02 sensor before? My truck is a 2001 3.4L V6 Manual.

First pic attached is the upstream sensor, next 3 are what I believe to be the downstream sensor.

 

P0420 is either a leak in your exhaust or a bad cat....usually its a bad cat. O2 sensors throw different codes when they go bad. I had a malfunctioning rear 02 sensor last year and it was a P0136 code, replaced the sensor and it went away. Year before that I had a P0420 that wouldn't go away, replaced my catalytic converter and it was solved.

 

How do I check for a leak in my exhaust? I just don't want to spend $600 on a new cat if I can just fix a leak with a gasket or something

 

Not sure as I've never had to check for one, but more often than not this code points to a bad cat so I would at least prepare yourself for that...people don't like to think of that as being the cause but it usually is, especially on these older trucks and the code pretty much spells it out. OEM cats are stupid expensive but you can get good aftermarket ones (Magnaflow) for way less. I got both my cats replaced with CARB approved Magnaflows for $600 out the door. If you don't need CARB ones, then they're even cheaper.

I imagine an exhaust leak could be easy to spot if you carefully look around the exhaust piping especially where the gaskets are, if there's a leak it's usually around there from old age. Just don't do this when the truck is hot, of course

 

Easiest way to check for exhaust leak that I have used:

- park outside
- add a can of Seafoam via the brake booster vacuum hose
- crawl under truck and see every exhaust leak you have very clearly

I did this recently with a p0420 code. I found the muffler leaking badly and the rear O2 mount rusted badly and leaking pretty good. I ordered a midpipe and muffler/tail combo from Summit for approx 175$, installed them(keeping the old O2)and no codes for 2 months/700 miles

 

What some people do is get a spray bottle with a soapy solution and spray any joint. Then they get a shop vac and discharge the exhaust like and air compressor into the tailpipe to get bubbles at leak! Exhaust leaks are common at the donut gasket on pipe and usually on top where it is not readily visible.

 

My P0420 was a bad sensor. I changed the sesnor and had no issues for 20k+ miles.

Being the easiest and cheapest, I'd start with the sensor.

 

If you do go the sensor route first, make sure you get a Denso sensor....Toyotas don't respond well to anything else and make sure it's a legit one. Lots of knockoffs posing as Densos on places like eBay and Amazon. When I got my rear o2 sensor for the P0136 code, I picked it up from Pep Boys

 

Its been a while since doing this but I believe the connector is either under your passenger seat or on top of the truck frame, between frame and body.

This might help:
https://www.youtube.com/watch?v=DhwTAosCJCc

 

yeah the rear O2 sensor harness goes up into the cab and under the passenger seat...kind of a pain to get to but not terrible. Think I replaced mine in about 45 min and that was taking my time. The trickiest part was that little metal plate that bolts under the sensor to protect it from rocks/debris...I just cut it off with an angle grinder lol

 

I used a 3\8th inch hose a couple feet long put it to my ear and the other end near all the gaskets while running that's how I found it sucking air in before the second sensor made it show lean. My carpet was cut already wear someone replaced the sensor before I got the truck.

 

make a homemade smoke machine. Lots of how-to videos on youtube

 

Thanks for all your help guys, I just got my denso 02 sensors both upstream and downstream so now I just have to figure out how to get this damn heat shield covering the downstream 02 sensor off. I don't have a Dremel or angle grinder or anything to cut it with, it seems like there has to be a way to remove it without cutting, why would Toyota make it so the only way to replace the sensor is by cutting the heat shield? Seems like they didn't exactly think this one through lol.

 

Yeah I had trouble with this as well, the heat shield only comes off if you remove the two bolts above it that hold both it and the sensor in place. Those bolts are hard to get to with normal tools above the heat shield.

The video I watched on Youtube, the guy used his angle grinder and just cut it off. I used my angle grinder to cut it 3/4 of the way off so I could bend it downward and access the bolts. Then I was able to put it back in place by bending it back upward when I was done. It has a cut through it but it still serves it's purpose and will be easier to remove in the future.

I would get an angle grinder if you can, or you could probably do it with a Dremel and a cutting wheel. It's pretty thin material.

 

I think there are 2 12mm nuts holding the heat shield and oxygen sensor. I just loosen the bolts holding the rear exhaust pipe to the cat and pushed the pipe back a little bit enough to have access to the nuts holding the heat shield and oxygen sensor.

 

They cut the carpet to get to the sensor connection? Damn that's brutal lol, it's easy to just move the plastic molding on the edge (2 or 3 screws) and then lift the carpet up.