MJP911

I have a CEL problem that is driving me nuts. Car is indicating codes P0136 and P0140 (P0136 porsche fault code 17 - o2 sensor aging behind cat bank 1;
P0140 porsche fault code 12 - o2 sensor behind cat conv bank 1) When I reset CEL, it stays off for a while, but then comes back. It usually comes back while traveling under light acceleration/coasting - sometimes within a few minutes of being restarted (eg after stopping for gas). I had all the O2 sensors replaced - same problem. One of the aftermarket mufflers from the PO was making strange sounds and blew out its packing so I replaced both of them too - still same problem. Funny thing is the CEL won't come on at the track or when driving briskly - only under light acceleration/coasting. Any ideas would be greatly appreciated. Also, if anyone has an electrical diagram for the O2 sensors etc, that would be appreciated - I suspect this is an electrical problem - maybe the signal or other wire shorting out.
Thanks!!!!

996FLT6

Your cats? Mike

MJP911

I think its more electrical based on the codes I'm getting.

Macster

Not a short. Aging related error codes are tripped because the sensors do not react fast enough or they do not vary enough in their voltage output.

One possible cause, ignoring the possibiilty the sensors are just old/worn out, is if the sensors fitted did not have the proper connector and the connector had to be removed and the sensors wired to an old but proper connector. If this splicing job done poorly... and solder should not be used since it can affect the sensor voltage levels, bias the levels one direction or the other.

If the sensors have the proper connector then nevermind.

Another explanation is then the sensors are just not very good.

Yet another is the converters are not working properly. I note the codes appear during light acceleration or coasting.

Coasting is kind of odd because the engine controller is open loop and not monitoring the #2 sensor output.

Much the same applies when the car is being driven hard like at the track. To meet the torque demands of the driver the DME can ignore/suspend #2 O2 sensor monitoring for awhile -- the fuel systems switch from closed to open loop mode -- so the DME can enrichen the mixture to meet the driver's torque demands.

As a result the DME may not be able to complete an O2 sensor and converter test. The problem still exists but the DME just hasn't gone far enough in its testing to reach a conclusion.

Not sure it applies, but I have noticed with one of my Porsches that if I use some gas other than Shell or Chevron (say I use Rotten Robbie or USA Gas, etc.) the check engine light is more likely to come on and the error code is always P0430 (converter performing below efficiency).

If I clear the code and then make it a habit to feed the car (mostly) Shell gas with an occasional (one tank out of 20...sorry but a Shell station just happens to be very handy and its prices less than a nearby Chevron station's prices) fillup at a Chevron station the check engine light stays off.

Thus you might see if changing gas brands (always run premium!) helps.

Sincerely,

Macster.

MJP911

Macster,
Thanks for the very detailed response. I think at some point there may have been a splice in the line. Based on your response, it sounds like a poor connection in this splice might be the cause of the problem. Alternatively, it may be an open circuit - either due to a splice or otherwise. Using Durametric, I got the following data:
O2 Sensor voltage behind cat conv bank 1 - 0.48V (varying)
O2 Sensor voltage behind cat conv bank 2 - 0.68V (varying)
O2 Sensor resistance behind cat conv bank 1 - 0
O2 Sensor resistance behind cat conv bank 2 - 128
The codes indicate a problem with the bank 1 sensor and the 0 resistance seems odd to me. I'm thinking this could be a true 0 resistance (short) or an infinite resistance (open circuit).

911SLOW

P0136

1. Check oxygen sensor IF OK THEN GO TO STEP 2

IF NOT OK
Replace oxygen sensor.
Erase fault memory, then road test vehicle
or perform short test.
Read out the fault memory.
No faults in memory.


2. Check wiring harness.
IF OK MOVE TO STEP 3
IF NOT OK
Repair wiring harness.
Erase fault memory, then road test vehicle
or perform short test.
Read out the fault memory.
No faults in memory.
End

3 Check output stage of DME control module. IF NOT OK
Replace DME control module.
Perform adaptation.

911SLOW

How to check oxygen sensor.


1. Heat the oxygen sensors (road test car under load
or run engine without load at high rpm).
2. Connect Porsche System Tester 2 or scan tool and
read voltage of oxygen sensor 1 after TWC.
Display: Voltage fluctuations between
0 mV and 800 mV.
➠ Oxygen sensor OK.
In the case of a display around 0 V, give several
acceleration bursts for approx. 10 seconds at
6 000 rpm. If 0 V is still displayed for 1 or 2 minutes,
a short circuit has occurred between the signal wire
and sensor ground.
3. Remove oxygen sensor connector 1 after TWC.
4. Connect voltmeter to pins 3 and 4 of oxygen sensor
connector towards the DME control module.
Display: approx. 450 mV.
➠ Replace oxygen sensor.
Display: approx. 0 V

911SLOW

How to check wiring harness.
1. Remove oxygen sensor connector.
2. Connect special tool 9637 to wiring harness (DME
control module connector).
3. With an ohmmeter, measure resistance between
special tool 9637, pin II/11, and oxygen sensor
connector, pin 3.
Display: 0 - 5 Ω
4. With an ohmmeter, measure resistance between
special tool 9637, pin II/17, and oxygen sensor
connector, pin 4.
Display: 0 - 5 Ω

911SLOW

How to check output stage of DME control module.
1. Connect special tool 9637.
2. Remove oxygen sensor connector 1 after TWC.
3. Connect voltmeter to pins II/11 and II/17.
4. Switch on the ignition.
Display: approx. 450 mV
If the display reads < 400 mV or > 500 mV,
replace DME control module.

911SLOW

How to Perform DME adaptation.
1. Switch on the ignition.
2. Wait one minute.
Do not press the accelerator.
3. Switch off the ignition for at least 10 seconds.
4. Read out the fault memory.

911SLOW

P0140

1 Check wiring harness.
1. Remove all four HO2S connectors.
2. Switch on the ignition.
3. Connect voltmeter at sleeve to pins 3 and 4 of HO2S
connector 1 after catalytic converter.
Display: approx. 450 mV
4. Connect voltmeter at sleeve to pin 3 and ground.
Display: approx. 720 mV
5. Start engine and run at high rpm for 3 minutes.
6. Connect voltmeter at sleeve to pins 1 and 2.
Display: battery voltage
If the measurement according to items 3, 4 or 6 does
not show the specified values, remove DME control module
connector and check wiring for continuity.
If no continuity can be detected, repair wiring harness.
If continuity exists, replace DME control module.


2 Check function of oxygen sensor.
1. Heat the oxygen sensors (road test car under load or
run engine without load at high rpm).
2. Connect Porsche System Tester 2 or scan tool and
read voltage of oxygen sensor 1 after catalytic converter.
Display: Voltage fluctuations between
100 mV and 800 mV.
àOxygen sensor OK.


3 Check oxygen sensor heating.
1. Remove HO2S connector 1 after catalytic converter.
2. Connect ohmmeter on pin side to pins 1 and 2.
Display: 1.8 - 2.5 W at 20 °C.
3. Connect ohmmeter on pin side to pin 1 and HO2S
housing.
Display: ¥ W
4. Connect ohmmeter on pin side to pin 2 and HO2S
housing.
Display: ¥ W

MJP911

Thanks, John - much apprciated!!!!!