CEL keeps coming back

[FlaglerMegacab4x4]

So I've had the same CEL on for the past few months now P0420. I already replaced the downstream o2 on bank 1 (drivers side) with a Bosh oem replacement since the code is showing bank 1 that didn't fix it. Next I found an exhaust leak on the exhaust flanges so I removed the oem hardware and replaced with fresh bolts lock washers and lock nuts on all four. The light came on shortly after I finished replacing them today. The truck just hit 84,000 today so I don't see how it could be a bad cat. I don't hear any rattling or feel any vibrations from the cat so I know it's not clogged and it doesn't smell funny either (heard it would smell like foul eggs). So any thoughts or suggestions would be extremely appreciated thanks.

 

[05daytona]

I'd start with checking the wiring to the O2 sensor as well as the quick connect.

 

[Redtruck-VA]

Looked up the code and this is what I found:
Symptoms
The main symptom to the driver is the illumination of the MIL (malfunction indicator lamp). You will likely not notice any drivability problems, although there may be symptoms. For example, if the substance inside the catalytic converter is broken or failed, it may be restricting the exhaust which will result in a feeling of reduced power output from the vehicle.

Causes
A code P0420 may mean that one or more of the following has happened:

•Leaded fuel was used where unleaded was called for
•A damaged or failed oxygen sensor (HO2S)
•Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
•The engine coolant temperature sensor is not working properly
•Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
•Retarded spark timing
•The oxygen sensors in front and behind the converter are reporting too similar of readings
•Leaking fuel injector or high fuel pressure
•Cylinder misfire
•Oil contamination

 

[FlaglerMegacab4x4]

thanks for the replies im gonna try replacing the sensors