Thread: Removing rear O2 sensor on 01-newer PCMs -- Things you should know

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

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

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

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.

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?)

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!

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

I thought this symptom was from the EVAP solenoid purging? Mine does this then goes normal. At least, I think it does.

I will try enabling the codes and just setting them to no-MIL and see how it does.

The evap purging is a different symptom and you will see the fuel trims go severly negative for no apperent reason which is really just the pcm compensating for the excessive fuel tank vapors that are being expelled into the engine. When the above mentioned condition happens the car will have perfect fuel trims driving down the road, in town, highway, it doesnt matter, and then out of the blue you will notice that the fuel trims start climbing up until they max at +16.4 and is never a negative number. It will stay locked at +16.4 indefinately until you give it a lot of throttle quickly or as Ryan mentioned above a high speed coast down. For the majority of the people that I have talked to about this problem their cars were stalling coming up to a stop light or it will badly surge and then cough out.

I have never seen my LTFTs positive. I guess my PCM doesn't have this issue. I need to go back in the PCM and see how I have the codes programmed. I know it doesn't throw the code as it never shows on any reader.

Turning the codes off in the tune only hides the fact that the rear 02 is not present or throwing a code but has no effect on the pcm running the cat diagnostics, you just never see a trouble code simply because it is turned off. This also seems to be hit and miss and there are many guys that have never had this problem while some have had the problem and dont know why or how to fix it.

Interesting info, regardless. Something to watch out for.

Is there any other way to fix it besides with a tuner? The only thing i have access to is a snap on graphing scan tool.

I was thinking today... are we sure this is 01+? This suddenly made sense to me as why O2 sims don't work on 04+ cars, because the PCM would notice the sim didn't change when it performs the diagnostic. If it was 01+ cars, the code should still be thrown on all 01+ even with the use of a sim, but it doesn't.

Well thats the odd part of the equation, it doesnt happen to all cars. Some guys with as little as an off-road non cat'd downpipe have problem or most guys that have modded cars with no cats see this. What I really think that is happening though is that HOW many people actually scan their cars continuously while driving every day to even know this is going on? Some cars wont show the bad side effects as much as others tune and mod dependant. Unless you have a scan gauge or something in the car and are watchin LTFT driving for a long time you may not even know the problem exists. An 02 sim will not cover this problem up because all it does is constantly switch the voltage to make the pcm think it is working properly. If the pcm throws more fuel at the engine for the cat diag to make it run rich it also wants to see the rear 02 follow this, a sim will keep bouncing the voltage around and not provide the data that the pcm wants to see.

You know what Im thinking? Fuel economy. Just how much gas would I save if this diagnostic procedure were not run? Every little bit helps.

On the cyclic scan of my Aeroforce I see my LTFTs every 10 seconds or so for 2.5 seconds at a time. Never noticed them go positive.