996 O2 sensor issue

[mbletsas]

I have a '99 996 C2 Cabrio with 114K miles on it.

The check engine light has been on despite my mechanic's repeated efforts (since the previous owner of the car).

I have bought a Durametric diagnostic kit and the car consistently produces the following error codes a few minutes after start:

moduleType = MotronicDME78

DTC Count = 6

Fault Number:P0151

Description:Porsche fault code 18 - O2 sensor ahead of cat. conv.

Repair Infomation:bank 2

DTC Count = 6

Fault Number:P0131

Description:Porsche fault code 10 - O2 sensor ahead of cat. conv.

Repair Infomation:bank 1

DTC Count = 6

Fault Number:P0137

Description:Porsche fault code 12 - O2 sensor behind cat. conv.

Repair Infomation:bank 1

DTC Count = 6

Fault Number:P0157

Description:Porsche fault code 20 - O2 sensor behind cat. conv.

Repair Infomation:bank 2

I.e. all 4 oxygen sensors report failure in the same manner (Short to ground/below lower limit/lean mixture threshold). I read in the forums that when this happens, it is usually a problem with the MAF sensor. I checked the intake, cleaned the MAF sensor as described elsewhere in the forums, however the errors keep coming back.

The MAF seems to be working fine. I took a 15 minute drive with the laptop attached and the MAF values varried from 12 kg/h to 150 closely tracking RPM.

The O2 sensor output on the other hand doesn't seem right. Both the pre-catalytic converter ones seem to randomly read between -0.2 and +0.7 volts while the post-catalytic converter ones seem to be stuck around 1 Volt.

Any ideas about what to try?

Thanks,

Michail

[Loren]

Things to check:

Short circuit in signal wire to ECM ground or housing ground.

Short circuit in signal wire to ECM ground or housing ground.

DME control module output stage short to ground.

Possible causes:

- Wiring harness

- Oxygen sensor

- DME control module

[mbletsas]

Thanks!

Do you by any chance know what the nominal voltage reading values of the O2 sensors are?

M.

[1999Porsche911]

Your MAF signal is way too low. . Check the connection and clean it.

Edited October 1, 2009 by 1999Porsche911

[mbletsas]

Your MAF signal is way too low. . Check the connection and clean it.

I have cleaned the MAF sensor and its connector. I run another data log session and recorded values ranging from 18Kg/h (800rpm) to 305Kg/h (4000rpm)

What doesn't make sense is the reading for the post-catalytic converter 02 sensors which is always around 1V

The car eventually lights the CEL with a P1126 (oxygen sensing adaptation) error.

M

[Richard Hamilton]

The oxygen sensing values aren't in volts. 1.0 is the Lambda value. It should be 1.0 +/-0.05.

Oxygen sensor voltages should be between 0.04v and 0.79v. What are the voltages of the O2 sensors after the cats?

[1999Porsche911]

Voltage for your O2 sensors can range, and should range all the way to 0.90+ volts. At WOT, even tho your O2 sensors do nothing to control fueling, your after cat sensors should rise from low 0.80 to 0.90 volts indicating proper a/f ratio. However, just because they might not have the proper readings, does not, and is most often not, an indication of bad O2 sensors. Your reading of 18 kg/hr indicates that the air mass value of the MAF at idle is correct.

P1126 indicates a lean condition. Remove your idle control valve and clean. Check for air leaks in the intake AFTER the MAF sensor, vacuum lines and throttlebody bolts as well as oil cap. Leaks in the exhaust before the primary O2 sensor can also throw this code as will low fuel delivery caused by low pressure or fouled injector(s).

If you have done any mods to the car and moved or played with the sensor wires, check them for kinking or stretching or damage. The sensor wires are as critical as the sensor is since the wires are used by the sensor to breath.

Edited October 3, 2009 by 1999Porsche911

[Richard Hamilton]

I would agree entirely with 199Porsche911. The important thing is that the voltages of the pre-cat sensors are jumping around rapidly, which suggests they are working correctly. In addition to what he suggests, I think it would be worth looking at the voltages of the after cat sensors, to see if one is sluggish, or reading a constant voltage.

[mbletsas]

The oxygen sensing values aren't in volts. 1.0 is the Lambda value. It should be 1.0 +/-0.05.

Oxygen sensor voltages should be between 0.04v and 0.79v. What are the voltages of the O2 sensors after the cats?

Correct, didn't notice that I was reading Oxygen Sensing. The Durametric tool doesn't give me the post-cat O2 sensor voltages.

Depending on the run, I have recorded pre-cat O2 sensor voltages that range from -0.17V (???) to 0.82V (car warm) and 0V to 0.89V (car cold)

Oxygen sensing value in the same conditions varried from 0.93 to 1.1 (car warm) and 0.94 to 1.2 (car cold)

M.

[mbletsas]

P1126 indicates a lean condition. Remove your idle control valve and clean. Check for air leaks in the intake AFTER the MAF sensor, vacuum lines and throttlebody bolts as well as oil cap. Leaks in the exhaust before the primary O2 sensor can also throw this code as will low fuel delivery caused by low pressure or fouled injector(s).

If you have done any mods to the car and moved or played with the sensor wires, check them for kinking or stretching or damage. The sensor wires are as critical as the sensor is since the wires are used by the sensor to breath.

I just read my codes after the latest run and noticed that after the P1126, the car also threw a P0446 (Porsche fault code 95 - Shutoff valve activated - charcoal filter).

To the best of my knowledge no mods have been done to the car.

When I cleaned my MAF sensor, I also cleaned the throttle body and checked the intake for leaks (nothing visibly wrong).

I assume the idle control valve is the Bosch part attched to the throttle body (on the left side when looking from the rear of the car).

Anything tricky in detaching it and cleaning it?

The oil filler cap doesn't seem to leak also, when removed with the engine idling you can hear vacuum (and feel sucking if you cap it with your hand). Is that normal?

Thanks for all the suggestions,

M

[Richard Hamilton]

Where are you located? If it is a RoW car, you may not have O2 sensors after the cats.

The tank vent valve is item 5 in the diagram. I don't believe it is a serviceable unit.

[mbletsas]

Where are you located? If it is a RoW car, you may not have O2 sensors after the cats.

The tank vent valve is item 5 in the diagram. I don't believe it is a serviceable unit.

I am located in Massachusetts and I do have 2 sets of O2 sensors.

M.

[Xabiitxu]

My apologies to recover this old thread, I'm just lost and desperate for some help.

 

I'm facing aparently a similar issue, but not sure it's the MAF.

I was to reflash my 996 C2 (MY2000) and first found max power was at 277 hp instead of the expected over 300 (bypass, gt3 throttle body, ipd plenum, bmc air filter...).

Then the guy checking the numbers found too lean mix at higher rpm which I did not expect as car was running perfect IMO, no fault codes nor dashboard lights. Anyhow we postponed the reflash till the lean issue was solved.

See the dyno bellow:

 

SInce this test last week I've clean the MAF, checked for leakages and read a LOT of posts about it but still could not get a clear idea. Since I have a durametric cable I did a couple tests on 3rd gear WOT a recorded the following parameters:

1-RPM
2-Mass air flow (MAF)
3-Adaption, range 2 (FRA), bank 1
4-Adaption, range 1 (RKAT), bank 1
5-O2 sensor voltage ahead of cat. conv., bank 1
6-Adaption, range 2 (FRA), bank 2
7-Adaption, range 1 (RKAT), bank 2
8-O2 sensor voltage ahead of cat. conv., bank 2


1          2       3      4     5    6    7      8
1.681 160 1,02 -0,19 0,84 1 -0,52 0,87
1.928 188 1,02 -0,19 0,88 1 -0,52 0,88
2.176 221 1,02 -0,19 0,86 1 -0,52 0,87
2.418 237 1,02 -0,19 0,88 1 -0,52 0,88
2.673 244 1 -0,19 0,88 1 -0,52 0,89
2.921 295 1 -0,19 0,88 1 -0,52 0,87
3.156 323 1 -0,19 0,87 1 -0,52 0,86
3.400 342 1 -0,19 0,88 1 -0,52 0,88
3.679 390 1 -0,19 0,87 1 -0,52 0,87
3.985 437 1 -0,19 0,87 1 -0,52 0,88
4.253 468 1 -0,19 0,87 1 -0,52 0,87
4.539 516 1 -0,19 0,87 1 -0,52 0,86
4.816 561 1 -0,19 0,86 1 -0,52 0,86
5.104 589 1 -0,19 0,85 1 -0,52 0,85
5.424 624 1 -0,19 0,84 1 -0,52 0,84
5.697 667 1 -0,19 0,84 1 -0,52 0,83
5.988 695 1 -0,19 0,83 1 -0,52 0,83
6.286 680 1 -0,19 0,81 1 -0,52 0,81
6.565 704 1 -0,19 0,81 1 -0,52 0,76
6.857 764 1 -0,19 0,83 1 -0,52 0,82
7.056 784 1 -0,19 0,85 1 -0,52 0,83
7.306 643 1 -0,19 0,84 1 -0,52 0,81


Any clues?

Thanks in advance

Edited June 27, 2016 by Xabiitxu

[Xabiitxu]

Some further info.

RPM vs MAF signal graph:



Doesn't it look weird at the 6,000 rpm mark? VAriocam, MAF or something else?

Thanks for your help