P0420 code

Hello all. I have a 2001 Tacoma pre runner. (3.4L, 5 speed automatic) 151,000 miles.
I’ve owned it for roughly a year. It has been flawless and I have kept up with normal maintenance. Lately, I’ve been getting a recurring code (P0420) It comes up every ≈350 miles. It reads something along the lines of “catalyst efficiency below threshold bank one.” It has a custom cat-back exhaust that I didn’t install but looks professionally done. Truck is fine otherwise. I’ve noticed my exhaust has smelled richer, and my mpg has declined. (Getting about 50 less miles out of a tank) (it’s also around 0 degrees where I live.)
Suggestions on how to move forward?
It sounds like it could genuinely be the cat, but I’ve heard to start by looking at the 02 sensors or even cleaning the mass airflow sensor first.
Thank you for any suggestions!

 

Check for exhaust leak


https://www.tacomaworld.com/threads/addressing-some-common-misnomers-of-the-p0420-code.396763/

 

At the top of the 1st gen forum there is a "sticky" section. In this section there is a post all about the P0420. Read it and if you have any questions after that dont hesitate to ask them.

 

Likely an exhaust leak.

 

Makes sense, thank you gentlemen. I’ll try to fix any exhaust leaks and go from there!

 

Po420 is low catalyst efficientvy. Standards set by EPA that even new cats under certain condition can not meet.

Exhaust leaks are most common starting point checks. The worst thing you can do for po420 is new sensetive o2 sensors. You want to "numb down" the sensors. Cheap o2 sensor spacers more often than not cure the issue completely. But only if your cat is still working 1/2 way decent. And it probably is.

 

As a PSA... The EPA got "smart" around 2008 ish and they forced manufactures to incorporate what I was told is an "crazy Ivan" strategy. Every so often the PCM will force a lean or rich condition for a fraction of a second just to see how ALL of the 02 sensors react. This is a pass/fail type strategy. A shrouded O2 sensor will not react to a known rich/lean condition and if it does this often enough, it will set a P0420 AND set a P0420 permanent code. A permanent code can only be cleared after fixing the problem and by driving a minumum distance AND having a certain number of drive cycles, usually 15 cylces without any codes being set.

 

Interesting

 

I know some people check for exhaust leaks with this trick:

- Using a shop vac, reverse the hose, so the vacuum blows air not sucks.

- Shove vacuum hose in your tailpipe with a shop rag around it to create an (airtight ish) seal.

- Turn shop vac on, spray soapy water on exterior of exhaust pipes and look for big bubbles.