08 Grand Prix P0420

Car: 2008 Grand Prix 3.8L V6 base model (118,000 miles)
Code: P0420 Catalyst System Efficiency Below Threshold (Bank 1)

First off, my Catalytic Converter was replaced in July 2015 and it operates at normal temp, roughly 1100 degrees F. 5 months with no P0420 after replacement.

I’ve observed my two O2 sensor ever since I’ve been getting this code. Mine upstream is a wave from .1 to .8 and my downstream is a constant line with minimal movement roughly around .75. My Short and Long term fuel trims are also reading normal. Long term is never more that +/- 5%, it usually settles at -3.3%.

After deleting the P0420 code, I will get around 7 to 14 warms-ups before it comes on again. Freeze-frame data always has it around idle when the code comes on – normal Cat temp – it doesn’t give me Fuel trim or O2 readings in this freeze-frame data.

Prior to replacing my Cat, the downstream O2 was a large wave (similar to the upstream), thus I replaced the Cat which steadied the wave, which it still is a good line.

Since battling this P0420 code I’ve replaced both the upstream and downstream O2 sensors (Denso), I’m still getting the same readings as described above. I also disconnect the battery to reset the Fuel Trim memory. The spark plugs are wires were replaced with the last 5,000 with ACDelco iridium.

The pictures attached are where the code reader is telling me the error is coming from (as far as I can tell). This says it’s a “Catalyst Test Bank 1 using Catalyst DFCO Exit Test.” I can’t find any information on what this means.

So my analysis is tells me all the reading are within reason and the Catalytic Converter is doing it’s job properly.

Any insight is GREATLY appreciated.