P0420 Code

Getting this code on my 2001 3.4 Tacoma 4 door. I have read it could be the O2 sensor or the cat.

Which 02 sensor should be replaced?
I have 2 cats,which one could be failing or both?

 

Check for exhaust leaks first. You can do this yourself with a shop vacuum. Take the exhaust hose off the vacuum and put in on the exhause pipe, seal it and spray a soapy solution around the O2 sensors and the exhaust connections and look for bubbles.

 

Usually P0420 is the O2 sensor which is downstream/after the cat. Either cat could fail but definitely check for exhaust leaks. Sometimes rust will create holes around the welds, etc.

 

So I'm basically blowing air into the exhaust?

 

After leak check possibilities are... well... exhausted..., gas cap seal is somewhat common. So much so Toyota has an updated gas cap. Its a good place to start. o2 extenders or antiffoulers help fight overzealous EPA programmed emmision levels. They will often elimate po420 codes or at minimum reduce them. These codes are common on weak, but not bad catalyst performance. This + curtain driving conditions can cause a cat "under performance standard" or low oxygen saturation po420 issues. All common Toyota po420 issues.

 

For what it’s worth, my 420 code was exactly what it said it was… bad cat. Did everything before new exhaust, o2 sensors the works. It was just a bad cat.

 

Yes, pressurizing the exhaust, this test works.

 

I am getting the weak catalyst as well. Im sure its the cat also but will check the o2 sensors first only in bank 1.

 

Weak cat does not mean bad cat. Maybe meating the over zealous EPA numbers that reality won't back. Even brand new o.e.m. cats can throw a po442/po443 under certain conditions.

Unless dead, o2 sensor replacement with new is actually worsening the issue as the readings are now MORE sensitive. The exact opposite of what you want. Weak cat + very active o2 = tattle tell trouble. Thats why o2 spacers tend to work.

Ever notice the guy that bah humbugs anything but a new cat usually aimed the "new o2" parts cannon at the repair first? Sure a new expensive cat will always work but why not try a cheap extender fix first?

Funny thing is these guys tend to buy aftermarket cats. A year or twor later the minimal metals cheap aftermarket use are worn out... often worse than the quality "weak" o.e.m. they thru out.

 

Got a chance to look at things under the Truck and found that the gasket between the rear cat and exhaust pipe has broken. There was a 1/4 gap missing out of the gasket. Going to replace that and see what happens.

 

In my case, the cat looked fine when I looked through the downpipe end but when I removed it completely, the other side was completely shredded up/cracked apart. I ended up putting a $300 walker cat in and it has held up a couple years so far.

 

Before or after the cat?
Ez enough fix exhaust leaks can and do set these codes?

Always fix the obvious first either way. Hope that gets it

 

It was the outlet on the rear cat that connects to the exhaust pipe.

 

I had a bad weld (exhaust leak) at this same location cause my P0420 code.

 

Hope it's whats causing mine.Going to get under the Truck tomorrow and do a little more looking around.

 

Toyota updated the gas cap as it was common evap related leak/issue. Cheap possibility. Think of it as maintenance.