2003 Tundra P0420 Code

Hi all! I'm new here, so I'm trying to get used to the site.
I have an '03 Tundra SR5 4.7 L that showed the P0420 code recently. A local exhaust shop replaced the left catalytic converter, and I replaced the O2 sensors with an upstream Denso and downstream Bosch. The check engine light came on again with the code again yesterday after about 100 miles and over a week of driving. The exhaust shop says I need to have diagnostics done because something else could be the problem. My mechanic doesn't now why the code would come up again or why diagnostics would be needed when it's specifically the 420 code. It's really frustrating. Any help would be appreciated.

 

Please start with Shifty’s FGT megathread. It has the answers you’re looking for. You probably didn’t need a new converter. Be sure your O2 sensors are Denso and nothing else. I recommend changing both upstream and downstream sensor at same time.

 

I have no idea how to find that thread.

 

Here you go. . .
https://www.tundras.com/threads/so-you-wanna-buy-just-bought-a-1st-gen-tundra-eh.115928/

 

Get rid of the Bosch and get a Denso

 

Denso O2 sensors.

What brand Cat did you get and is it a cat that is applicable to your truck/state? Only a select number of aftermarket cat brands are worth buying. Also, where the truck was sold new matters. If you live in Texas with a truck that was originally sold in California with a California spec cat, your truck needs a California spec cat for the truck to not throw a cat related code.