Rossi

Just got my GTS back from the major repair, front of the car was removed, so was the front subframe as we did front main seal, upper oil pan reseal.

One week later, about 400 miles later .... just popped up CELs P0421 & P0431.

Anyone successfully addressed these MILs?
I am aware these codes are typically related to Oxygen sensors (the rear ones???) or catalytic converters.

Any input would be greatly appreciated.

Rossi

Anyone can confirm the location of which O2 sensors the code is related to, is it the downstream set after cats?

deilenberger

"P0431" is a "standard" OBD-II code (anything under P1000 is) meaning you can easily google for it. Give that a try and let us know what comes back.

Rossi

Did some searching and and with my situation of both codes popping up after major repair, it is very probable that front (upstream) and rear (downstream) oxygen sensors were interchanged. I will call my Indy tomorrow and report back.

kadn33

In my 955 the pre-cat sensor is a wide-band ( 5 or 6 wires in the connector, not sure) and the post-cat is narrow-band ( 4 wires in the connector ). If the same applies to the 957 you should be able to visually track the looms going to pre or post cat side from their connection in the engine bay, just by the SAI pumps supports, and find out if they are swapped.
Red and black connectors top right corner in this picture courtesy of pelicanparts.com, same on the passenger side:

Rossi

I just spoke with the repair shop, They did not touch the O2 sensors, I might not be on the right track I guess.

kadn33

Then maybe is time to scan the O2 sensors behavior and patterns as well as the cat converters operating temperatures.
If not already familiar with the sensors there are many Youtube videos dealing with the way the system works and how to isolate faults after plotting their signals that would prove useful. I found NGKTechnical or Schrodingers Box videos very good but there's a lot more.

Rossi

Thank you kadn,

I scanned at idle, anything jumps out?

Rossi

This is at idle, one bank at the time, no steady stream for rear O2s. Don’t these look interchanged? The rear ones (red) show way more action.

kadn33

First of all let me say that my experience with O2 sensors is limited to my vehicle which never had a code triggered or a malfunction, so all my opinions are based in pure theory from the videos I watched and minght not be accurate.

You want to make sure that a drive cycle is completed or at least that the cat converters have reached opearting temeperature from driving for a while before plotting the sensors. All the videos I watched recommended as well to do the readings at an engine speed of 2000 - 2500 rpm. I guess this is for the same temeperature reasons and to force the cat converters to work.

From the plots what I can see straight away is the Bank 1 Sensor 2 voltage jumping from 0.1V to 0.7 V while it should be sort of steady ( like Bank 2 Sensor 2: 0.6V min and 0.7V max ) Thats either the O2 sensor gone or the catalytic converter not working but again you want to try at higher engine rpm to confirm this behavior.

Any chance you can check the temps before and after the catalytic converter with an infrared thermometer? This might help isolate the converter if faulty as the outlet temp should be higher compared to the inlet when working fine.
Otherwise, swapping Bank 1 Sensor 2 with Bank 2 Sensor 2 would allow to see if the fault moves: voltage variation goes to bank 2 - sensor faulty, or voltage variation remains - cat converter faulty, although this requires access to the sensors first and that´s not easy.

If there was an issue with the pre-cat sensors that would mean theres a problem with your air/fuel mixture but that doesn´t seem to be the case. Any engine running issues you noticed?

Finally, did you try to clear the codes? I guess you did already but since it is not mentioned....worth a try I guess.

I´ll watch again the videos to see what else can be checked and how but I am afraid that with the basics I know I am not able to help you further right now.

Rossi

Thank you again kadn,

Yes, I did clear the codes initially and they came back. I will run the scanner with in the recommended RPM range, the car drives actually already nice, no harshness, starts beautifully. No issue. You would not know there is anything wrong with it. Scanner also reads cat temperatures, and many many more functions. Anything else I should report?

kadn33

Not sure about what else to check. It is suspicious that after working in the area you get codes for both sides: things don't usually fail in pairs on isolated systems. Try with higher rpm and maybe that will show a clearer output graph from the sensors.
The goal of the inlet vs outlet temp check of the converters is to make sure they do their job while the scanner will probably give you a working temp only but again, hard to believe they failed at the same time shortly after service.

Eventually and if everything else seems to be good, an inspection of the wiring condition would help as well.

Rossi

This is at 2100 RPM. Bank 1 looks better. It seems that it runs rich right?
If I look at the black line for pre-cat sensors, it seems to be running way too high right?
It suppose to flex between 0.1 to 1, it is pretty flat at 1. It seems to be all screwed up.
The upstream sensors supposed to flex in values, downstream sensors should be nearly flat is my understanding.

Rossi

This is what my catalyst monitors reported:
=> bank two FAILED
=> bank one barely PASS


ED

kadn33

I just went through the thread in renntech and pretty much agree with the last recommendations.

Just wanted to add that from the O2 sensors plots you posted the Bank 1 post-cat O2S12 is the one fluctuating the most and I would expect that cat converter failed however you then refer to the Bank 2 cat as failed instead.
In any case at this stage you don't really know whether is the cats or the post-cat sensors failing and before spending big money on cats I would still recommend checking their in and out temps if the Techron and others don't work.

Lastly, I plotted my sensors (2004 CS with 90.000 miles / 145.000Kms) and at 2000rpm the post cats were initially waving approximately between 0.2V to 0.7V and later stabilized at around 0.5V. At 3000rpm the voltage was 0.6V but stabilized as well. Once stabilized the voltage drops and rises were of +- 0.050V max, for reference.
The lambda remained at 1 at all times which makes sense because the ECM will always aim to keep it there.

Regards.