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

**Sinister Performance** · 12-01-2011

Originally Posted by darkhorizon

I dont see any reason to get involved with a open/closed loop discussion.... But my suggestion is someone used to OBD1 / map sensor setups would probalby find it is much easier to run open loop on a maf sensor.

What are you implying? I've tuned hundreds of OBD2 PCMs so I don't see what your point is.

As far as the "different climates" and "some guy came in with a bad open loop tune" goes... I have never had issues with temperatures, elevation change, humidity, or anything hurting any of the 15+ cars I have put on open loop.

That you know of. Do all those people who's cars you've locked into open loop check the status of their tunes on a daily basis to see how well that open loop tune is matching up?

2 of the fieros I have done in michigan were shipped out to moutian areas, one in PA the other in colorado... both with widebands and they both know how to use them, neither have had to adjust their tunes...

Like I said before, if the customer has the ability to adjust the tune themselves and wants to keep a constant eye on it, then giving them a "locked in open loop tune" is certainly a "choice". However, I don't think I would base all of my reasoning on just 2 instances where it appears to be "working". I bet any other tuner could take a look at how these 2 cars you speak of are running with their open loop tunes and find something off in those tunes.

Also every open loop tune I have done will command no more fuel than about 15.1 in most cases, and at cruise it usually averages a 16:1 air fuel....

That's good on the valves and the valve seats.

Also, professional tuners like ourselfs have a few other tricks that we normally do to enhance closed loop operation with bigger cams... The laymans out there reading the stupid "tuner guide" are just going to throw a pile of crap at their maf and hope for the best.

I tune everything so the fuel trims operate within a reasonable range of as close to 0% as possible. I try to leave the closed loop and fuel trim learning enabled in all my tunes so the PCM has the ability to adjust for changing conditions. And conditions do change. I've seen it happen countless times on stuff that has been here at my shop and on scan logs customers have emailed me. While the fuel trims ultimately don't change all that much (over the year, as the weather changes), they do change some. And I would rather allow the PCM to have the ability to make the adjustments it needs rather than to just allow the engine to run stupid lean while going down the highway. Forcing overly lean AFRs on the engine for extended periods of time is not good practice if you want your engine to last; especially at the loads it is subjected to run under when cruising on the freeway.

Of course if you don't care about changing engines every year or two because you can find them for $100 at the junkyard (and you don't mind wasting a weekend to swap it out), then I guess engine durability is of little concern. How many engines have you tuned that have blown head gaskets, chucked rods out of the side of the block, or otherwise failed? Be honest...

**darkhorizon** · 12-05-2011

On cars I have tuned there has been 1 broken L26 rod after insane amounts of abuse, and my drag car blew a felpro headgasket this year on 27psi of boost.... maxing out 2 340lph fuel pumps and 110# injectors.... made about 30 passes on one 40 mile trip from detroit and it blew.

My track record is pretty good if all I have to show is a broken $9 headgasket from "bad tuning".

**blown383z28** · 12-12-2011

one more question.. I'm not sure if i remember reading this earlier in the thred..

y wont a o2 simulator work for the rear? wont it do the test and then read the rear o2 and the sim will make it look good??

**Sinister Performance** · 12-12-2011

Originally Posted by blown383z28

one more question.. I'm not sure if i remember reading this earlier in the thred..

y wont a o2 simulator work for the rear? wont it do the test and then read the rear o2 and the sim will make it look good??

The cat diagnostic the PCM runs tests the oxygen storage capacity of the catalytic converter. When this test is running, the PCM will command the A/F ratio rich for an "extended" period of time to see how long it takes the rear O2 sensor to report a rich condition (when it was previously reporting a lean condition). If you look at the scan from a completely bone stock car with a functioning cat, you'll notice the rear O2 sensor signal is very slow to respond to changes in AFR compared to how fast the front O2 sensor responds. And the reason why is because the design of the catalytic converter is set up so it stores oxygen when the AFR is lean and releases it when the AFR is rich.

O2 sensor sims output a continously varying rich/lean signal which used to work on older generation OBD2 cars. These worked fine before GM added the advanced form of the cat diagnostic (starting in 2001 model year). The problem is the PCM knows when it is commanding this cat diagnostic to run and if the sim isn't outputting the exact signal the PCM is looking for when it runs this test, the test will fail or will not complete.

Basically you would need a "smart" O2 sim in order to work around the cat diagnostic. You would need the sim to watch the pre-cat O2 sensor signal so it could detect when the PCM runs the cat diagnostic test so it could feed the proper signals to the PCM on the post-cat O2 sensor signal circuit to make the PCM think there is a properly working catalytic converter installed. So far I have not seen any such device.

-ryan

**blown383z28** · 12-13-2011

thank you for the good info!!! thats what i was looking for! I'm going to leave the 00 computer in my 01 car and just buy the credits for that computer.. i figure the lose ill take on the 01 computer i bought the credits for i can some what make up by bringing the cat in for some cash and save on all the head aces!!

**darkhorizon** · 12-13-2011

You havent even said what OSID you have yet... We could probably get your 01 file fixed if you would just give some information like we have asked.

**blown383z28** · 12-14-2011

Originally Posted by darkhorizon

You havent even said what OSID you have yet... We could probably get your 01 file fixed if you would just give some information like we have asked.

Im sorry i must have missed that part about the OSID.. not a big deal.. I sold the 00 car with the 01 computer in it last night. I had 2950.00$ in the car and i sold it for 3250.00$ so i dont feel so bad about the 100$ in the 01 computer.. plus ill take the cat in off the 01 and get about 50-80 for it..

**LapeerGTP** · 02-17-2012

Ok can I get the cliff notes version of this thread for my 04gtp. I'm putting headers on and removing the cat. I'm purchasing hptuners to do all my tuning needs but I'm concerned about the cat diagnostic test. Has hptuner come up with the ability to disable it or is it not a big deal if I leave the downstream o2 in?? Sorry for the newbie question but I don't wanna spend all the money only to have the car run like sh*t

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**Sinister Performance** · 02-17-2012

Originally Posted by LapeerGTP

Ok can I get the cliff notes version of this thread for my 04gtp. I'm putting headers on and removing the cat. I'm purchasing hptuners to do all my tuning needs but I'm concerned about the cat diagnostic test. Has hptuner come up with the ability to disable it or is it not a big deal if I leave the downstream o2 in?? Sorry for the newbie question but I don't wanna spend all the money only to have the car run like sh*t

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I have HP Tuners and last time I looked at an 04 GTP file you still could not disable these diagnostics using their software. If you can't get someone to find you the same OS with these tests deleted (using TinyTuner), then there is something else you can do.

You can choose to leave that post-cat O2 sensor installed or you can remove it. But no matter what, you're going to need to set all the DTC's relating to the catalytic converter and post-cat O2 sensor to "No MIL" and make sure the "SES Enable" box is unchecked for all of them. Doing this in HP Tuners will allow the codes to set but should prevent any check engine lights or warning messages from being displayed on your dash - and prevent the diagnostic tests from constantly running.

**LapeerGTP** · 02-17-2012

Ok sounds good. I'll have to do some more reading on tiny tuners.

Thanks for the quick reply

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**moddman** · 02-20-2012

Did not see a post showing how its done(Cat Diagnostic Disabled) so this is to help the newbie out. Just zoom in on pic after you save it to your pc. Set the cat enable temp to 20 C*. Set tests per trip to zero.

**matt5112** · 02-20-2012

Wouldn't you want to set it to something you'd never achieve to disable it?

**Bio248** · 02-20-2012

set it to 'no report' and discontinue caring what values it sees. it'll never set a code.

**Pure2sin** · 02-20-2012

Originally Posted by Bio248

set it to 'no report' and discontinue caring what values it sees. it'll never set a code.

IIRC that will only make the car run like crap because of it still running the cat test but not setting a code. It will still make the car go lean because it's trying to do the test. If you raise the temp it won't run the tests and there are a few other options in TT for taking it out. You have to get all of them though.

**Sinister Performance** · 02-20-2012

Originally Posted by Pure2sin

IIRC that will only make the car run like crap because of it still running the cat test but not setting a code. It will still make the car go lean because it's trying to do the test.

Every car the cat test runs on I've seen run RICH when the diagnostic test is active, not lean. Rich as in 11:1 AFR or somewhere around there.

**Pure2sin** · 02-20-2012

Originally Posted by Sinister Performance

Every car the cat test runs on I've seen run RICH when the diagnostic test is active, not lean. Rich as in 11:1 AFR or somewhere around there.

I meant rich not lean..brainfart.

**wa04compg** · 04-01-2012

If you keep the rear o2 sensor in place it will run the test then it shuts off after a min or so and goes back to 14.7...I am catless with rear o2 in place and I have not had any issues

**LapeerGTP** · 04-01-2012

Originally Posted by wa04compg

If you keep the rear o2 sensor in place it will run the test then it shuts off after a min or so and goes back to 14.7...I am catless with rear o2 in place and I have not had any issues

I plan on leaving it in because its a new o2. I have a dhp so I should be able to delete it of necessary.

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**Sinister Performance** · 04-02-2012

Originally Posted by wa04compg

If you keep the rear o2 sensor in place it will run the test then it shuts off after a min or so and goes back to 14.7...I am catless with rear o2 in place and I have not had any issues

That seems to be what happens if you leave the Rear O2 installed because the PCM sees a result (signal) from the rear O2 when it runs the test. Now whether or not this happens the same for all model years is a question.