I encountered the same issues (and same P1126 code IIRC) with my 2.5L this past summer and tried many of the same steps you tried like cleaning, disconnecting the MAF, checking for vacuum leaks, O2 etc.

I know you said you checked the intake already, but its worth checking again between the airbox and the screen just downstream of the MAF (assuming the S's intake also has a screen).

FWIW, the culprit on my car turned out to be a clear piece of plastic wrapper caught inside the intake on the screen inside the intake tube just downstream of the MAF. I'd checked the intake previously but I didn't find it at first since the plastic was clear and not very large (maybe 1" -1.5" square) and it might've been clung to the side of the intake.

Hi! thanks for the suggestion, I will definitely look into that. If I'm being perfectly honest, other than checking that tube for smoke during my smoke test, I didn't investigate the intake tube very closely. As soon as the temps get back above 30 (ugg!), I will take a look and report back. As usual, thanks all for your time!

Hello friends. Sorry for the delay in updates, the weather has not been cooperating.
Yesterday was the first break in temperature so I scoped the intake tube as suggested and there were no obstructions. I moved on to replacing the fuel injectors on Bank 1 (immensely easier than on Bank 2). I actually noticed some positive change in the cold idle, throttle response and general feel of the motor overall. The engine felt noticeably smoother on the test drive and performed much like I remember it performing before this all began. In addition, fuel economy seemed to improve, but it was based on visual gauge estimation, so not data.
This is not the end of the road though. The car stored a P1126 during the test drive and there is still a momentary "swoosh" when sharply pressing the throttle.
It looks like the weather will be improving again next week sometime, so I'll check the cats by removing the fore O2 sensor and checking back pressure on each side. I'll also try smoking the intake again. I also noticed that cylinder 1 appears to have begun leaking oil at the spark plug tube, so I'll be replacing all 6 of those as well (they're on their way).
I do have a question though regarding P1126. Can anyone give me the correct verbiage for P1126 on DME 7.2? I realized my Durametric is reporting it as P1126 - Porsche fault code 356 - Multiplic. mixture adapt. lower load range B.1: and the Bentley manual reports it as P1126 Porsche DTC 35 - Oxygen sensing area 1 cylinders 4-6 - Short to B+/above upper limit/rich mixture threshold. Is P1126 a Bank 1 or Bank 2 code on 7.2? Any help there is appreciated!
I'm also planning to do a "hard reset" and capture some fresh data. Can anyone with a Durametric and DME 7.2 tell me what the short term fuel trim entry is? I was under the impression it would be "oxygen sensing Lamba" or "fuel trim mean value" but I do not see either in the list of Actual Values.
As usual, thank you for your time and help!

Hey fan, late to the party but I have a couple of thoughts...

Intake backfire is a lean condition (so long as timing is not completely wrong, which would be impressive), and if your o2 sensors are reading rich at that time if be would be very suspicious. A new set is not expensive, it may be worth a try.

Are you still pulling vacuum at your oil fill tube? May be worth double checking - i have read (not personally experienced though) that if you lose vacuum to aos then your crankcase pressure will build up and the you will notice this when the plug tubes start to leak oil.

Occam's razor - fouled o2 sensors reading too rich, engine pulls fuel in response, lean condition causes intake backfire (at idle only as the system will run richer under load), fries AOS tube, created vacuum leak, lean condition is now worsened and plug tubes are leaking oil. New injectors may have helped band aid the problem but I don't think they were the chicken or the egg.

Anyway, just a thought.

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Hello and thanks for joining in! I really appreciate the ideas. I've tried to use as few assumptions as possible while troubleshooting, so I like the application of Occam's Razor, but I'm not sure the data matches the assertion. I'll try to explain my approach below.

I have considered just changing the O2 sensors many times since this started as they're so cheap, but I resisted because according to Durametric, it all appeared to be working as intended (to me at least). The FORE cat O2 sensors were responding with a sine wave and the fuel trims seemed to be adapting by removing fuel at idle and adding it under load. I have never been able to get them to "peg rich" or "peg lean". Now, considering your comments, for the O2 sensors to be working properly, wouldn't it make sense for them NOT to be operating as a full range sine wave UNTIL the trims have adjusted? Do O2 sensors have a failure mode like this?

Shouldn't they be indicating either rich or lean through altered sine amplitude until after the adjustment? I have data that shows the O2 sensor output is essentially unchanged even as the RKAT value is changing. Some insight here would be invaluable. Perhaps it's because the amount of trim being added isn't that large (RKAT ~1.5-3.0 and FRA is ~1.25)? This is part of why I'm trying to find the short term fuel trim within Durametric, to compare with the O2 sensor output.

Looking back, I have specifications from Porsche I have cobbled together from this site and others and it looks like the O2 sensor range is 0.04mV less than or equal to "O2 Sensor Volts" less than or equal to 0.79 mV. I do have O2 output data with some values above 0.79mV, however in ScannerDanner's youtube videos, there are times he has said this doesn't matter much.

Can anyone confirm that 0.79mV is the proper hard limit for O2 output using Durametric Tool for a 986 01S?

Regarding the crankcase pressure, I've tested with a manometer and I am pulling -5.05 inches of water column, which is what it appears I should be getting. I honestly think the oil on the valve cover at the spark plug tube (this oil residue is new) is likely due to O-ring failure but, I'll put the manometer back on the oil-fill tube next time the weather cooperates.

I would be overjoyed to simply replace the AOS and the O2 sensors and be done, but I've done my best to avoid changing things that "appear" to be operating properly. If I can nail down an analysis that shows the O2 sensors are not performing correctly, I'll replace them.

Additional questions, what should the "Engine Load %" be at idle in Durametric? My data seems to indicate 19.00-20.25 at idle? Is this correct? If not, what does this point toward?

I'm open to any ideas on further troubleshooting! Also, I have no problem replacing the O2 sensors if the values above 0.79 mV indicate they're failing. Thank you all for your continued help!

Sorry but I really can't comment on the specifics about the durametric readouts. My knowledge of afrs is limited to use of wide band o2 sensors and afr or lambda readouts, and narrow band sensors like ours can be very finicky. With that being said though my experience in working on cars has been two things 1 - it's really easy to overthink these types of problems 2 - you can spend forever looking up detailed specs, reference values, etc when that time may have been spent turning wrenches. I'm not knocking your approach at all, but rather criticizing all the time I have spent doing that exact thing when all I really needed to do was start physically working through things on the vehicle. The theory behind ICEs is really quite simple, and all things revolve around a few basic principles that will lead you to a diagnosis in nearly all circumstances. (I'm sure you've heard it before but don't lose the forest through the trees lol) I'm totally with you that the "eh let's replace this part and see what happens" approach is fool hearted at best but i trust the processes of reason and elimination more than anything when it comes to complex problem solving. Not to mention that Amazon's return policies are great, so try a new part and if it doesn't fix the problem clean it up and send it back and then let someone else buy it at a great discount lol.

Anyway, I'm happy to help if I can. Good luck and keep us posted!

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Hey, thanks for the quick response. I've got to admit, I am beginning to be tempted toward just replacing a few things (especially the low-dollar stuff) and see what happens.

I think if I can't get specifics on where the O2 output signals should be or what the STFT PID is in Durametric, this limits my ability to continue troubleshooting with any reliable effectiveness. But I cannot find any confirmation that maintaining a full-sweep sine wave of 0.04mV to 0.85 mV is even a failure mode for narrow-band O2 sensors, meaning logically, they're working and it's something else. That said, after I test the cats with a back-pressure gauge, replace the spark plug tubes, test crankcase vacuum with my manometer again and smoke the intake once more, my next step may be replacing the O2 sensors. Fortunately, I enjoy working on the car and learning more about it, so I'm just going to keep at it.

Thanks for the time and the help!

From what I see in your O2 signal sine waves that you have posted in this thread in post 13 your O2 sensors are fine they show a typical sine wave.
The issue I see is your post Cat O2 signals are showing that your Cats are not working properly. So the Catalytic converter are suspect the sensors are fine.

Sorry I just went through the thread again and see in post 17 that you ran the O2 sensor data again at a more steady rpm.
But to me your bank 2 post O2 cat sensor still shows that your cat on bank 2 is suspect. maybe not failed but boarderline.

Hey blue do you think a failing cat would cause his problems though? I could see that if a cat were clogged and not flowing well that would cause loss of power because of choking down flow. Would out cars attempt to lean out the mixture in order to reduce emissions if it senses a bad cat? I'm not asking to disagree, it's purely from ignorance I've not ever dealt with a bad cat before and I'm trying to piece together how that would play into what he's dealing with.

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Hello and thanks for the follow-ups. Yes, I intend to back pressure test for failing cats when the weather cooperates, probably early this upcoming week. As far as I can tell however, the post-cat O2 sensor does not play with the ECU, so I don't think that could be causing the P1126, but maybe if the cat is failing, that could cause it?

I'll add results as soon as I perform the test. Thanks for the continued interest and inputs!

Agree your P1126 is another issue.
Perhaps some form of vacuum leak.
Or MAF sensor but there are ways to test it.

Yes, I agree. As you pointed out though, the downstream O2 sensors do look like something is up, so I won't ignore them.

The MAF appears to be operating properly, 15 kg/hr at idle and the Voltages looked about right last time I pulled data. I pulled some data on my test drive after changing the fuel injectors but I was in a rush and I'll need to pull more. Will pull MAF, O2, Fuel Trim and Engine Load data next test drive after I test cats, smoke the intake and replace the spark plug tubes.

Thanks again!

A test I do is compare MAF signal to Throttle position sensor signal.
There is a direct correlation between the two.
I hook up my Durametric and put the MAF signal and the Throttle position sensor signal on the same Graph. Then I take the car for a good long drive.
I set the laptop on the passenger seat so I can watch the two signals.

What you should see is the two signals paralleling each other. The TPS signal is slightly smoother then the MAF but by very little. That just the difference between air movement generating one signal and mechanical movement generating the other signal.
So you should see Both signals following your foot so to speak. If your very smooth on the throttle both signals should increase with the same smoothness. If you floor it both signals should react the same way. If your smooth on the throttle or holding a steady RPM both signals should follow each other. If you see something erratic in the MAF signal when your throttling smooth or holding a steady RPM then there is an issue with the MAF.

I do this with my car over a 52 mile loop. Gives me every driving condition one would ever encounter. City stop and go, to country flat out, hills, curves the works.
Actually a fun test to do.

This sounds great. I'll include throttle position sensor output in my data pull the next time I am performing a test drive.

So, I've been doing further reading on the pre-cat O2 sensor because now I'm stuck on the fact that the output of those sensors does not appear to change while the fuel trims are being modified (ie. in Post #14, the Bank 1 and 2 pre-cat (FORE) sensors display almost the same sine wave while the RKAT is at 0 as it does when RKAT moves to -0.25 to -0.75 on all the way to -2.5). If the ECU is adjusting the fuel trims negative as a result of O2 sensor feedback loop reporting rich (my understanding of the function), shouldn't the O2 sensor display a sine wave that looks like it's seeing rich? I'm considering inducing a LARGE vacuum leak at idle while pulling data and see if the O2 sensors respond to this by indicating lean, at least for a couple of seconds. ScannerDanners videos show lots of response from the O2 sensors while he is changing various inputs (pressing the gas, inducing vacuum leaks, plugging vacuum leaks, etc) and my sensors don't seem to display any of that. Is this specific to the operation of these sensors? Maybe they're just bad and 200 bucks will fix these issues? I really need to see STFT to better understand this I think.

Again, any input is greatly appreciated. Thanks!

Hi all. Just a brief update, following the advice of ike84, I checked Amazon for the Bosch oxygen sensors. I'm not sure why, but I haven't purchased parts for this car from Amazon before. Anyyyyyyway, the sensors have full refundability and were at a lower price point.

Plan now will be to swap in new O2 sensors when I pull them to back-pressure test the cats. I will perform an overnight "hard-reset" (poor-mans handover) of the ECM the night before and, provided cats pass the back-pressure test, go for a test drive and collect a good deal of clean data with (hopefully) known-good cats and new upstream O2 sensors. I'll report back following these efforts.

As usual, thanks all!

Happy hunting! Keep us posted

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1. I do not see a failing Cat causing his problems.
2.Our cars do not attempt to lean out the mixture in order to reduce emissions if they sense a bad Cat. Post Cat sensors on cars of that era do one thing = monitor Cat health via reading oxygen levels.
The only thing a narrow band O2 sensor of that era can do is sense oxygen and only in a very narrow band.
3. The reason I pointed what I see as possible failing or bad Cats is that he was talking about replacing his 02 sensors. I was pointing out that based on what I see in the O2 sensor Sine Wave, that the Sensors are fine. But the post Cat signal is suggesting a failing Cat.. So no need to replace the O2 sensors.

Ummm I think that's it :D Hope I answered your questions.
And there is nothing wrong with disagreeing how else could one learn he was wrong;)

So I need to go and watch scanner danners videos about cats because my knowledge of them is still very much comic book version. When they fail what is the main symptom? Do they choke down the exhaust and kill power through reduction of flow? Or do they just stop cleaning up the exhaust and result in no other difference? I remember newarts video of how he toasted his cats when he blew his engine but that's the only other time I've ever heard of them going bad.

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Yes, ScannerDanner and also Schrodingers Box on youtube have great videos on fuel trims, O2 sensors, failing cats etc. ScannerDanner is a professional and Schrodingers Box is a DIY guy, so the videos are different, but both focus on the mathematics and science of engine controls.

From what I've learned watching their videos it appears that your former statement is what occurs. The failed cats typically impede the flow and result in very poor engine response and a lot of hot start issues. That said, I see exactly what blue62 was referring to with respect to my downstream O2 sensors. With the car at a steady rpm, a properly functioning cat should result in a downstream O2 sensor with a very flat output. My downstream O2 sensors do not look like they are doing that. Hence, a back-pressure test, to see if the cats are clogged or failing ... or at least that's my understanding from watching ScannerDanner and asking questions of my mechanic buddy.

Cats fail in a number of ways.
1. Physical damage such as hitting speed bumps. Can lead to a rupture,hole, leak in the case.
2. "Plugging up" usually caused from things like too rich of mixture over time. rings not seating causing oil blow by, leaking head gasket. Antifreeze gets into the exhaust.
3."Burning out" caused by to lean of mixture, excessive idle time, things that create to much heat.
A burned up cat has no back pressure issues or the like, it acts just like a good cat except it doesn't do it's thing.
Those are the things I am aware of.

They are designed to last the life time of the car. But they function in a very narrow range. So fuel air ratio needs to be very close to 14.7 to 1. I think somewhere around one half of one percent + or - of 14.7 to 1. Outside that range for extended periods then they burn up or plug up.

So something to try and keep in mind when dealing with OBDII and the P-Codes.
Its not about engine performance or power or gas mileage. The primary function of the OBDII system including the sensors is Catalytic Converter Performance and longjevity. Everything else is secondary.