Tuned on an '05 Monte SS L67 the other day. Owner installed a ZZP DP to delete the cat and he also removed the rear O2 (even tho there is a bung in the DP).
Anyway he brought it to me for a tune. Went in a disabled the CAT diagnostic code as well as the rear O2 diagnostic codes. Went for a test drive and discovered that after driving the car for a couple of minutes the ST and LT fuel trims would both max out and the car would go pig rich and stay there.
Talked to TrannyMan since he has heard of this before and found out this problem seems to exist with only the 01-up PCMs. This got me thinking, and then I remembered back to my GM OBD-2 training at the dealership -- reading about this diagnostic self-test the PCM runs to determine if the rear O2 and cat are both working as desired. Basically the computer commands the fuel trims rich for a while and expects to see the rear O2 signal to eventually go rich after the cat gets saturated.
Well, as it turns out if you remove the rear O2 and disable the trouble codes for it, the computer seems to still want to run this diagnostic but will never stop running it. I had the wideband plugged into the car I was tuning and the AFRs were ranging from 10.5:1 to 11.5:1 pretty much all the time unless you floored the gas or decelerated from a high speed. So obviously this is not good.
I went back into the programming and turned the trouble codes back on for the rear O2 (but instead set them to "No MIL" just so the check engine light wouldn't turn on) and did another road test. Same thing happened again, the rich condition came back as the diagnostic test ran but this time it only lasted for about 20 seconds before the computer set a P0140 code (rear O2 not enough activity) and then the fuel trims and AFRs returned to normal for the remainder of the drive cycle.
I think what this diagnostic may be called is a "Cat Diagnostic". On some older vortec truck cals I have the ability to tune, I can actually disable this diagnostic. So now I am working with my tuning software vendor to see if we can add this ability to 01-up L67 and other V6 applications. Just thought I would pass this info along to you guys for reference. Of course I will let you know what we come up with.