Thanks for using JA,
I have pulled the check list for the p0420 code and found it to do the same thing that you are doing and check the O-2 sensors and ensure they are reading within specs. The next thing it says to check is the Cat converter and I am sending you the check list and hope it helps. Please if this info helps click on accept.
Did you perform the Diagnostic System Check-Engine?
Go to Step 2
Go to Diagnostic System Check - Engine Controls
Is the HO2S 2 voltage parameter as active as the HO2S 1 voltage parameter?
Go to Step 4
Go to Step 3
Important: The test may need to be completed up to 6 times in order to pass or fail.
Does the scan tool indicate DTC P0420 ran and passed this ignition?
Go to Diagnostic Aids
Important: Verify that the three-way catalyst (TWC) is a high quality part that meets the original equipment manufacturer (OEM) specifications.
Visually and physically inspect the catalytic converter for the following conditions:
Did you find a condition?
Go to Step 8
Go to Step 5
Visually inspect the exhaust system for the following conditions:
Did you find and correct the condition?
Go to Step 9
Go to Step 6
Visually inspect the HO2S 2 for the following conditions:
Go to Step 7
Replace the HO2S 2 sensor. Refer to Heated Oxygen Sensor 2 Replacement .
Did you complete the replacement?
Notice: Refer to Three-Way Catalytic Converter Damage Notice in the Preface section.
Replace the catalytic converter and the exhaust manifold. Refer to Catalytic Converter Replacement and Exhaust Manifold Replacement in Engine Exhaust.
Did you complete the replacements?
Did the DTC fail this ignition?
Go to Step 10
Observe the Capture Info with a scan tool.
Are there any DTCs that you have not diagnosed?
Go to Diagnostic Trouble Code (DTC) List