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.

-ryan

__________________
1987 Pontiac Fiero 3800 Series 2 Intercooled Turbo (12.26et @ 113mph)

www.gmtuners.com

Great information, stickied so we wont loose it anytime soon.

__________________
Scotty Hall
Bartlesville, OK
2001 Pontiac Grand Prix GTP
It whines a lot, but I like it.

The only limitation you will ever face in life is yourself!

I'm running into this issue with a friend with an '03....I'm gonna see if this helps he mpg's out.

Thanks for the info!

__________________
GreenBastardPerformance2000 GTP Coupe (the green bastard)
XP, Pacesetters, LS1 TB & MAF,MSD System, P&P LIM, P&P GenV w/ 3.2, 3.0, 2.8, Devil'sOwn Injected 100% Methanol,PowerTuned, TripleEdge Trans w/3.29's
12.92 @ 105mph

Maybe this is why my LTFT's have been off for so long...... It will need to be set back to non-reporting though if you want to pass emissions

__________________
2002 GT...Wizardair, ZZP 2.5 o/r dp, Special Eddition two-tone leather interior added, HUD added, 12" Brake Upgrade, ZZP strut tower bars, gmpp sway bars,-6an braided SS fuel hoses , Hptuned, Top Swaped.....
I've got Boost!! I've got only 1.5* KR left my poor L36 block.....

Got the updated software today and I tried it out on the '05 Monte. IT WORKS. The cat diagnostic does not run at all with the new settings (disabled via coolant temp qualifier and maximum number of tests allowed per drive cycle set to 0). The fuel trims work normally and AFRs are right where they are supposed to be (and nothing locks full rich ever like what it was doing). I was also able to disable all rear O2 codes so those would never set either.

I am now working with my tuning software vendor to get these parameters added for all 01-up applications supported.

__________________
1987 Pontiac Fiero 3800 Series 2 Intercooled Turbo (12.26et @ 113mph)

www.gmtuners.com

As far as I know, HP Tuners software does NOT have this ability yet (if that's what you are using). All they allow you to disable is the COT (cat over temp) diagnostic which is NOT the same thing as this cat diagnostic I am talking about.

So I assume that the DHP won't either then......damn.

__________________
GreenBastardPerformance2000 GTP Coupe (the green bastard)
XP, Pacesetters, LS1 TB & MAF,MSD System, P&P LIM, P&P GenV w/ 3.2, 3.0, 2.8, Devil'sOwn Injected 100% Methanol,PowerTuned, TripleEdge Trans w/3.29's
12.92 @ 105mph

I have been having this problem with my 01 GTP and could not figure out what was going on until i read this post. Now i just need to find a way to disable it

__________________

Would a rear O2 Sim allow the test to complete and prevent this from happening? If so it might be a good option for those without access to your specific tuning software (What do you use anyway?)

__________________
Cheap, Fast, Reliable... you can only pick two

Hey Ryan you may want to chime into my thread on HPT forum about Codys GS with the changes you made. I posted a reply to you finding the cause and it was with TC but it will have more credit coming straight from the horses mouth!

Quote:

Would a rear O2 Sim allow the test to complete and prevent this from happening? If so it might be a good option for those without access to your specific tuning software (What do you use anyway?)

An 02 sim does not fix this problem. I know this because the car I had trouble with already had one on it that works fine and removing it made no changes and the condition actually lessoned with nothing in place

__________________
www.TripleEdgePerformance.com
4T65E Transmission Info
Voted GPF.net Community Hero