P0420 trouble! Help!

I was recently told by a trusted muffler shop that both of my cats were bad in my 2001 tacoma. I replaced them both and I still got the dreaded check engine light. Code p0420. What else could be the problem? Thanks in advance!

 

Exhaust leaks between the O2 sensors, one or both of the O2 sensors, catalytic converters. Since you replaced the cats and assume there are no exhaust leaks since that should have been taken care of when the cats were replaced that would leave the O2 sensors as the most likely culprit.

 

That code is for the Bank 1 Catalytic Converter isn't functioning properly. Check your 02 sensor (one before the first cat). Check your fuel injectors. See if they are leaking or just need replacing. Also, check your spark plugs. Also could be improper ignition timing. But check the small stuff first.

 

What brand cats did you use as replacements? Sometimes cheapie cats just won't satisfy the ECU. Do you have access to and OBD2 reader that shows live data?

 

I don't have access to an OBD2 reader that shows live data. What exactly would that tell me?

 

Also what signs should I look for when I "check" the first o2 sensor? Or do I need to just change it regardless since I know the cats are new?