Strange P0420 issue with 2017 Tahoe 5.3

[abefroman]

My girlfriend recently had a new piece of exhaust pipe welded in after the cat, after discovering a hole in the factory pipe. About a week later, she's getting a P0420 code. Truck has about 120k on it. According to the torque pro graphs, the cat would look to me as it's mostly doing its job and only seems to deviate from the bank 2 cat performance under moderate to heavy acceleration. At idle and cruising at same speed they are nearly identical. I was thinking there might still be an exhaust leak somewhere but figured I would ask here to see if anyone has seen something like this before. I don't want to replace the cat until absolutely sure it is that. Thanks

 

[Fless]

Note the deep notch on B1S2 (driver's side) from time to time, comparing it to B2S2's graph. While it might be the CAT, and depending on the age, condition and the brand of the downstream O2 sensors, I'd replace them first to see if the graph improves.

It would be wise to look for exhaust leaks, too, especially around the manifolds.

I'm sure others will have additional comments.

 

[abefroman]

We did just replace the downstream o2 sensor with a GM one, still same issue. I did look for broken manifold bolts on the Driver's side but didn't see any.