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Need help in diagnosing P0133 and P1128 - replaced O2 sensor and CEL c


ebbjak

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So I looked up the codes and based upon my findings, I replaced both pre-cat O2 sensors.  In addition, I cleaned the MAF.  I cleared the codes and after about 30 miles the CEL came back on.  Same code.  What's up with that?  Is it possible a new O2 sensor is bad?

 

Welcome to RennTech :welcome:

 

Let's start with what year and model is the car?

 

On a 5.2.2 DME car, P1128 is indicating an overly lean condition in the bank housing cylinders 1-3.  Unfortunately, on cars equipped with the 7.2 DME, it is entirely different, so we need to know the year and model.

 

Ahsai has an excellent point, if you have access to a Porsche specific diagnostic tool, you can see the real-time voltage patterns which will show how the sensors are working.

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If you have an Android phone, get a $15 Bluetooth OBDII dongle and the Torque Pro app and you can plot graphs like these

http://rennlist.com/forums/996-forum/810606-obd-ii-code-reader-recommendation-2.html#post11299728

If you have an iPhone, get the wifi OBDII dongle. I'm not familiar with iOS apps but I know they have something very similar.

The above assumes you don't want to invest in Durametric, which is the best and most comprehensive for DIYers.

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Sorry.  You're right I should have included year.  It's a 2001 Boxster S, 38000 miles.  I will check the air intake after the MAF, thanks for the tip.  I don't currently have access to the Duramatic but I do have a smartphone, but how would that help? 

 

OK, you have a 7.2 DME, so the same code (P1128), which is an overly rich mixture, which could be a bad MAF signal, fuel pressure too high, a leaking injector, or a stuck EVAP purge valve.

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If you have an Android phone, get a $15 Bluetooth OBDII dongle and the Torque Pro app and you can plot graphs like these

http://rennlist.com/forums/996-forum/810606-obd-ii-code-reader-recommendation-2.html#post11299728

If you have an iPhone, get the wifi OBDII dongle. I'm not familiar with iOS apps but I know they have something very similar.

The above assumes you don't want to invest in Durametric, which is the best and most comprehensive for DIYers.

 

I ordered  the dongle today. I'll measure the O2 sensor readings early next week and report back. Thanks for the tips!  I didn't find any leaks in the air intake by the MAF, but I tightened all of the clamps and verified all of the connections to ensure they are snug.  I reset the CEL and will check the O2 readings when the Dongle arrives.

 

 

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This is interesting. Were these taken at idle with fully warmed up engine?

O2 1x1 is bank 1 precat sensor and it looks ok. O2 2x1 is bank 2 precat and it looks very lazy so it's a suspect to me. Both postcat readings look fine. The voltage is measured by the obii scanner on-board (shows 12v above) so it may not be accurate due to the cheap hardware so you better check with a voltmeter. Expect 12.8 to 14v with a healthy charging system.

What's interesting is both of your codes are complaining about bank1 and have nothing to do with bank 2. What are the LTFT on both banks as reported by Torque?

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If the LTFT are so close to zero, they are fine. Can you monitor the STFT on both banks at idle right after you drove and stop the car but just letting the engine idle? Say for a few minutes and watch the STFT? If they are very positive, that may mean you have a vacuum leak (or low fuel pressure). If they are very negative, that may mean you have a high fuel pressure.

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First of all, thank  you for all of your help.  I've had my car for 10 years and I've never had any issues with it, so I've still got a lot to learn.  I ran the STFT and as one might expect, it varied pretty dramatically.  The highest values were about 5.7, however almost right after that it dropped to -5.7 (Bank 1).  Bank 2 was similar.  The number that came in most often was .78 for bothbanks.  aert03.jpg

 

The number most often displaying was .78 for both banks.  My guess is that the numbers look relatively normal. 

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You're welcome. Just trying...not sure if it really helps. A few +/- % for STFT looks normal to me. The only thing that jumps out is really the bank 2 precat sensor swings very slowly. Normally it should cross the 0.44v about once a second during idle. Actually even bank 1 precat signal looks slow.

I want to make sure this is not due to low sampling rate. Could you plot ONLY the precat sensors on both banks again to get the max sampling rate? If the bank 2 precat signal still looks similar, I would suggest changing the sensor.

Also, is the CEL currently ON? Can you reset the codes one more time and see if they come back?

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I couldn't figure out how to plot what you were asking for so hopefully the pictures below will make sense.  I have reset the light today but have only put about 10 miles on since resetting.  The next couple of days my schedule will not allow me to get the car out to further test.  Also, both pre car sensors are brand new from Pelican Parts so I would guess it would be odd if they were both bad.dvolco.jpgdnk6ed.jpg

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Excellent plots. The small dots show the actual samples so we are sure these have high enough resolution. If you count the number of times the signal crosses 0.44v, it's about 15 times in 25 seconds in bank1 and 15 times in 20 seconds in bank 2 and the signal amplitude is correct also. In other words, bank1 is swing slower than bank2. If these were plot under similar conditions, then indeed the bank1 sensor appears to act slower so that could be the problem.

If the same codes return, I would suggest swapping the two new sensors and see if the codes follow the cuurent bank 1 sensor.

Edited by Ahsai
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Thanks for the heads up about the change.  If the light returns, I'll swap the sensors and see if the plots follow.  Both sensors are new, so other than a bad O2 sensor, what would cause it to be slower?

Yes but read the codes first before you swap them and then reset the light and retest. I cannot think of any other reason.

Also, you are sure the parts are correct for your engine, right? Bosch?

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