-15 is running a bit rich. What engine we working with?

Yeah black soot on the oxygen sensors is an engine running rich. -15 fuel trim tells me that it is running rich and trying to pull back fuel to correct. This is probably what killed the first cat. This might be a leaky injector or not enough air or somesuch.

How is your air cleaner?

Any custom work on this like a programmer or aftermarket parts?

What are its normal driving experiences? Grandma driving to the local Piggly Wiggly once a month? . . . Normal mix of cold, hot, traffic, freeway, etc.? . . . Driven like it was stolen? . . . Driven like it was rented?

Owner is 81 years old..... he drives like an "old man"........if there is a leaky injector, would it idle and run fine?

Malibu LT 2.0

Depends on how leaky. It can compensate a little. the -15 fuel trim would seem to show that it is compensating for excess fuel already. We're also just speculating about the leaky injector. It could still be something else, but the symptoms naturally point us there because there aren't many ways for one of these to run rich like that.

How is the air cleaner?

Any custom work on this like a programmer or aftermarket parts?

Nope, nothing..... the only thing was the P0420 that came on.