Have a 2007 Traulblazer with a 5.3. Cat efficiency test is causing misfires at idle and its getting really annoying at this point. What is the best method to stop the PCM from doing the test? Just disabling the P0420 and P0430 codes? Or is there more I need to do?

 

Second O2 sensors are constantly checking cat efficiency by comparing pre and post O2 levels. I wasn't aware there was a separate efficiency test.

 

There is. I'm not sure what models/generations actually perform the test, but if you're logging and the AFR suddenly spikes to super lean (16.7) and then after a few seconds drops down to extremely rich (like 12.3) the computer is testing the cats for their response to a change in AFR.

But, after doing a log and seeing a huge uptick in misfires on my Trailblazer, I'm thinking that my issue has expanded beyond the cat efficiency tests. Will be posting a new thread here in a minute.

 

Oh, you mean these guys? Yes, they exist in your calibration... E67 OS ID 12611938 although it would be a first ever seeing them cause a misfire at idle.

 

It is very noticable when my P59 running my SBC 383 runs those same tests. Mine does not really misfire but it does lean surge when the PCM leans it out at idle. The easiest thing to do is leave the rear 02s in place. I am out of luck on that one though without adding rear 02 sensor wiring as my 97 never had rear 02s as built by GM.

You must be running an older version of HP Tuners. A lot of those emission related switches have been removed in newer versions of the software.

 

Yes I keep old versions going way way back just for this purpose, I've just never seen them misfire before from these tests.

 

ive tuned hundreds of gen 3 and 4 and the first time i ran into this was actually tunning my own daily driver swap with a p59 ...it was super confusing the afr going all over. could feel the engine get weak.i alwyas thought it was my imagination,finnally threw the laptop and wideband in.. i realized there was a timed pattern..then i learned about this test running..all my rear 02 and evap etc had been shut off like i always do , so it was not at all on my radar. after a bunch of searching and multiple answers, including, "impossible to remove without changing o.s" to "disable rear heater codes" nothing worked...then i discovered somthing.. i enabled one rear o2, ses light disabled, walla... it cant run the test if it thinks one of the o2 sensors is dead.. works great ,, funny enough this is the only stockish l.s i own in my daily work truck and funny enough a well known tuner did the pcm years ago while i was busy with other things ,, 4 years later i finnalyy got a round to reall cleaning up the tune and found this first thing..probly cost me 200.00 in gas to figure out while logging.. hope this helps some one...

 

Ill give that a shot! So far nothing has worked. Thank you!

 

I can't imagine what sort of 1997 anything from GM wouldn't have rear O2 sensors from the factory. Those were all OBDII vehicles, which means they definitely came with rear O2s originally, unless perhaps you have some sort of HD diesel truck or commercial equipment like a Kodiak/Topkick.

Great info here.

 

Express van that has been family owned since 1998. No rear 02s as built from GM. Where the rear 02 subharness should attach it has always had a factory plastic block off cap. #2 in the factory parts diagram is the GM factory block off plug that threaded into the 02 bungs where the rear 02s would have resided in a California model. The van never had evaporative emissions pressure monitoring either. With the old black box PCM years ago, the fuel pump o-ring disentigrated and was leaking for who even knows how long at one point before I found it changing the fuel pump.




Some were also built fully open loop, completely lacking 02s and no cats as well. Factory GM parts diagram on a catless, open loop only 2002 model I ran across. I still have a copy of the factory 0411 open loop only tune file from this one as well. This one was a former US Forest Service van.


I am going to try his method as well on mine. See if it makes the P59 stop doing to 02/Catalyst testing. I can watch mine shift the commanded air fuel ratio from 1.00 to 1.1 then to 0.9 during the testing. It will run for about 60-90 seconds, both leaner and richer. I have gotten to where I will not even datalog a P01/P59 until it has about 3-4 drive cycles and 40-50 miles since the PCM was flashed because a lot of the data is rubbish until it has gone through its self testing and learning procedures.

 

^ Sounds like a commercial chassis exhaust configuration, likely for a 3500 Express/Vandura or bigger HD trucks like 4500-8500 series.

 

Would you mind sharing the file?

 

My van that lacked rear 02s was a 1500 L31 with the factory 3" dual to the muffler system. The catless was a 3500 L31 and it was Y to a single to the muffler. I also parted out a 1996 or 1997 K1500 Suburban that still had the factory exhaust and it was also built without rear 02s. This is the stock GMT400 C/K 1500 exhaust diagram. #9 is the same GM 22509274 02 delete plugs that were on that K1500 Suburban I parted out.

 

That is interesting as I have definitely seen 1500s (especially truck/suburban) from that era WITH rear O2 sensor and normal OBDII emissions monitors active. Light duty/passenger gas engine chassis was not usually exempted from normal OBDII emissions equipment in 1996+ in my experience.

But there are different emission-related RPOs for different sections of the country (for initial sale), perhaps that has something to do with it.

 

I also added the diagram for the C/K. #9 is 25509274 which is an 18mm plug to delete the 02s on vehicles that did not use OBD2 catalyst monitoring in that time frame. I looked and it had numerous OE applications beyond trucks even. A lot of the 2500HD/3500 6.0L/8.1L GMT800s also never had rear 02s.

 

Right; 2500+ makes sense as that's entry-level HD and includes commercial applications.

I could see this being the case for a commercial or HD truck/van/etc. chassis, but what else beyond that? Even B4C, 9C1 Special Service Package had catalyst monitors active, like all gas engine passenger cars of that era.

 

I would also pose a question..since theres very litle info i can find about this cat test, if any one knows the conditions for running it i would like to know.
in my p59 ecm wich was originally from a 3500 6.0 van , once i figured out the lean/rich swings , and i knew what i was dealing with, it was semi predictable.. test would run after some amount of temp and steady cruise, i could get it to do it predictibly within about 10miles of steady cruise. extreme throttle changes or stopping would be the only way to disrupt it.it would only run once every key cycle and would go 4-8 miles,..i dont know if the conditions where time based, milage, or?
also wanted to add at one point way before this, i tuned a 5.3 swapped vehicle with a p59 and it kept swinging like this on me ,i could not understand why it would be perfect {and sensible} then i would take it out and have to make wild swings to get it right...i maf disabled it at my wits end and that fixed it , admitantly a crutch but i now understand why..it disabled the test also..not neccesarily wrong inmop because in a lot of swaps the maf is either unnecasary or not wanted.. just another thought.

 

I have read in the past that GM actually had the EPA on them for some of the stuff they did in the 96-97 model years at one point. A lot of the 96-97 was only halfway implemented OBD2 wise. I worked on a lot of B/D body cars in the early 2000s. I saw a bunch of 94-96 9C1 Caprices here with factory test pipes. At one point GM sold the catalyst delete test pipes over the dealership counter for those cars too. The civilian cars had cats but the 9C1s did not. I put a full catless factory dual exhaust from a 9C1 Caprice on my 1993 Fleetwood. I had ZZ4 heads on it and used the LT1 manifolds. I have never been under a B4C car. I do not remember the Z56 Tahoe my buddy owned having rear 02s. If it had them it never set codes when he put long tubes and catless exhaust on it. I eventually tuned that one with TunerCats OBD2, may still have the stock tune file for it to see if it had rear 02s and catalyst monitoring enabled. The Vortec 350 and 8.1L both had over the counter GM parts for an EGR delete too at one point. Parts used on production vehicles.

 

These were factory GM parts on some of the 90s 9C1 Caprices. They had GM and a part number stamped in each side. I want to say were GM 12504014 and 12504015 or something like that. They bolted directly in place of where the cats were on the civilian cars. I saw a bunh of 9C1s with them here in Texas back when all of them were retired and became dirt cheap LT1 V8 powered sedans you could buy for $1,500. I do not recall the 4.3Ls ever having the factory test pipes or dual exhaust so it was an easy way to tell if the car actually had the 5.7L back in the day. A lot of those cars got banged up, put back together and you would have a 5.7L emissions sticker on a 4.3L car and vice versa when the front clips were swapped.

 

I've seen nothing like this in my area. Lots of 9C1 Caprices have crossed my path in the last 25-30 years, all were PPVs in the northern IL area for local municipalities, I've never seen one (even pre-'96) with a factory test pipe. B4U civilian models and 9C1 PPVs had the same factory dual exhaust configuration in every example I've ever seen in my region. My area also had L99 models as PPVs; single exhaust on those but no test pipe either.

B4C was the same, even 3rd gen (1992 and prior) B4Cs I've seen came with a cat (standard config) or two cats (N10 dual exhaust) in this area. Same for 4th gens.

I do wonder how much of this depends on the emissions related RPO spec'ed on the initial order sheet? Maybe some regions vary?

I had a 1996 Fleetwood as well, one of the last ones off the assembly line (built in 11/96, special ordered by a family member, one of the latest VIN numbers to be documented). I recall the D-body being slightly longer than a B-body, but I don't recall if the D-body exhaust was extended or if the tailpipes just terminated sightly farther behind the bumper (vs. B-body).