[mike 96 ws6]

I recently bought this truck and I'm trying to figure out what's up with the PCM.
The check engine light is non-funtional but the PCM will set DTC's. A couple of weeks ago I had a fuel pressure/injector issue and it set codes P0135, P0155, P0171, P0174. After I made the repair and cleared the codes it hasn't set any more since. I'm wandering if there is another issue needing repair that the PCM is not setting a DTC for. The truck runs pretty good but it has some issues such as lack of power at part throttle sometimes, and erratic transmission shifts.
The previous owner removed the cat and replaced it with a straight pipe but I'm getting no DTC (P04xx ect.). As you can see Bank 1 Sensor 2 (post cat O2 sensor) fuel trim is pegged @ almost 100% on my scanner. I suspect this may be due to a faulty sensor or wiring, but I would think the PCM would set a code.
I just purchased this Autoinginuity and I first thought maybe my scanner (Generic OBD2) was missing data, but I've had 2 other scanners on it and the data stream/DTC staus is the same. I ordered the Enhanced GM Expansion software from Autoinginuity today so I'll be able to post more info when it gets here.
Thanks for any help!
Mike

[matt167]

PCM has been tuned out for rear o2 detection.

[LATECH]

I would have to say that the software is corrupted, or as matt suggested tuned out,
I also noticed for a warmed up engine the Idle speed seems to be pretty hi.Intake air temp is also very Hi, are you in arizona? is it summer there? Hmmm, maybe you had the air intake sucking air from behind the radiator?I also see a small imbalance in the B1 +B2 fuel trims from side to side ,the 02 sensor voltage is close, even though the trims are close enough to be fine(within 5% in either direction), as long as the O2 are toggling back and forth from lean to rich to lean to rich etc.. it is OK.
Realizing this is a still frame display and not being able to watch data live being switched, I have to point out what I see as a concern.Thats all.

[T-bucket23]

Something is definitely weird. I don't understand your graph labels. The down stream O2 sensors have no effect on fuel trim thy are just there to make sure the cat(s) are working.
Is there anything plugged into the rear O2 sockets?

[LATECH]

Also, does the Check engine light illuminate during bulb check?(when key is first turned on)
MAP value looks low too, You may have an intake leak as well as a few other things.

[LATECH]

It may take 3 complete drive cycles for the PCM to trip a p0420-p0430 BTW.
Conditions have to exist for the monitors to run. Usually cold start to warm up, and roadspeed driving of 25 mph or more could be required to run monitors,as this constitutes a drive cycle, or trip, in many cases.
you should be able to make one trip, after all the monitors are running, to generate a 1 trip code , or pending code, which is a quicker process .

[mike 96 ws6]

Originally Posted by T-bucket23

Something is definitely weird. I don't understand your graph labels. The down stream O2 sensors have no effect on fuel trim thy are just there to make sure the cat(s) are working.
Is there anything plugged into the rear O2 sockets?

I just realized that. That 'sensor 2' fuel trim on my scanner should not even be there.
Yes, there is a sensor in the post-cat socket. (The configuration on this exhaust system is a pre-cat sensor on each bank, then 2-into-1 y pipe, then 1 cat and 1 post-cat sensor.

[LATECH]

Originally Posted by mike 96 ws6

I just realized that. That 'sensor 2' fuel trim on my scanner should not even be there.
Yes, there is a sensor in the post-cat socket. (The configuration on this exhaust system is a pre-cat sensor on each bank, then 2-into-1 y pipe, then 1 cat and 1 post-cat sensor.

That is another reason to suspect a corrupt flash program. If the cat is missing, or ineffective, the downstream O2 sensor will read pretty much the same as the upstreams.
The way the PCM knows that the cat is bad or missing, is that it compares the upstream value against the downstream, it should be different in the downstream, usually predominantly lower, and it should only move rich to lean about 1 time for every 3 times the upstream changes.
The fact that the downstream trim value is pretty much off the chart, and the voltage value actually looks OK, suggests the pcm algorithms are wacked, or there is a HUGE Exhaust leak between the up and down stream sensors, but my guess is you would hear that. LOL

[mike 96 ws6]

Originally Posted by latech

I also noticed for a warmed up engine the Idle speed seems to be pretty hi.Intake air temp is also very Hi, are you in arizona? is it summer there? Hmmm, maybe you had the air intake sucking air from behind the radiator?I also see a small imbalance in the B1 +B2 fuel trims from side to side ,the 02 sensor voltage is close, even though the trims are close enough to be fine(within 5% in either direction), as long as the O2 are toggling back and forth from lean to rich to lean to rich etc.. it is OK.
Realizing this is a still frame display and not being able to watch data live being switched, I have to point out what I see as a concern.Thats all.

This scanner data is during sitting in my driveway with my foot on the accelerator for an extended time so it's a little misleading.
I have noticed i may have a problem with long term fuel trim bank 2. (I'll record and post a link to a data log video tommorrow).
Yes the MAP value is low and I've looked and looked for a vaccum leak but I haven't found one yet.

The DTC for post-cat O2 sensor may be disabled via tuning - i don't know.
I use LT1_Edit programming software for my 96 trans am ws6. That software provider does not have software for the 2003 4.3 L. We tried their 1997-2007 LS1/LS6 Edit but there were too many compatibility issues so I returned it (thanks Ken & Dave). So I never got a chance to see what, if any, parameters have been modified in this program.

And no the check engine light does not illuminate when the key is first turned to on position.

[mike 96 ws6]

Originally Posted by latech

or there is a HUGE Exhaust leak between the up and down stream sensors, but my guess is you would hear that. LOL

I did hear a moderate leak in that area yesterday when checking the physical status of that post-cat sensor.

[T-bucket23]

Originally Posted by mike 96 ws6

I just realized that. That 'sensor 2' fuel trim on my scanner should not even be there.
Yes, there is a sensor in the post-cat socket. (The configuration on this exhaust system is a pre-cat sensor on each bank, then 2-into-1 y pipe, then 1 cat and 1 post-cat sensor.

Thought the cats were removed

[mike 96 ws6]

Originally Posted by T-bucket23

Thought the cats were removed

I'm describing the original factory exhaust system configuration which is presently intact and funtional other than a straight pipe in place of the cat. Sorry for the confusion.