Fault codes PO131 PO151

[Lamplighterson]

Hi all.

Getting fault codes on my early 996.

P0131 Oxygen Sensor Ahead of Catalytic Converter (Cylinders 1 - 3) - Signal Wire Short Circuit to Ground or Incorrect
P0151 Oxygen Sensor Ahead of Catalytic Converter (Cylinders 4 - 6) - Signal Wire Short Circuit to Ground or Incorrect

My 996 only has 2 O2 sensors rather than the 4 on later models.

Getting an irregular idle.

Clearing codes and waiting for car to cool results in 2 minutes on non codes (whilst the car heats up) followed by the irregular idle and then the codes reappear.

Thoughts? Dirty MAF sensor perhaps? Strange for both sensors to show fault...

[Richard Hamilton]

A dodgy MAF usually gives lean or rich limit fault codes on both banks, such as P1123/P1125. I would check the wiring and sensor heater relay.

[Lamplighterson]

Many thanks. Now trying to find location of the heater...

Edited by Lamplighterson on Monday 22 April 19:49

[Rockster]

Lamplighterson said:

Many thanks. Now trying to find location of the heater...

Edited by Lamplighterson on Monday 22 April 19:49

The heater is in the sensor.

[Lamplighterson]

Rockster said:

The heater is in the sensor.

Ah. Of course.

Would a wiring/heater issue trigger both codes though (on both sensors) if only one was at fault?

Seem to recall that when one reports a fault they all do?

[Rockster]

Lamplighterson said:

Hi all.

Getting fault codes on my early 996.

P0131 Oxygen Sensor Ahead of Catalytic Converter (Cylinders 1 - 3) - Signal Wire Short Circuit to Ground or Incorrect
P0151 Oxygen Sensor Ahead of Catalytic Converter (Cylinders 4 - 6) - Signal Wire Short Circuit to Ground or Incorrect

My 996 only has 2 O2 sensors rather than the 4 on later models.

Getting an irregular idle.

Clearing codes and waiting for car to cool results in 2 minutes on non codes (whilst the car heats up) followed by the irregular idle and then the codes reappear.

Thoughts? Dirty MAF sensor perhaps? Strange for both sensors to show fault...

My info is if P0151 is displayed any other faults attributed to the other sensors only one sensor may be at fault.

The P0151 code signals the sensor voltage level is below 150mV.

Likely the 02 sensor heater is bad. The other sensor could be replaced just on general principles. It is not ideal for one sensor to be new while the other one old even if the old one is not showing any signs of wearing out.

[Lamplighterson]

Thanks. Appreciate the help. As there are only two sensors on this car I take it they are both "pre" sensors (ahead) of cat rather than "post" sensors?

Want to make sure I order 2 of the correct original parts.

[Rockster]

Lamplighterson said:

Thanks. Appreciate the help. As there are only two sensors on this car I take it they are both "pre" sensors (ahead) of cat rather than "post" sensors?

Want to make sure I order 2 of the correct original parts.

My info is those models with just 2 sensors the sensors are ahead of the converters. These sensors are used by the DME to know it has control of the fueling.

The 2nd pair of sensors (absent in your car) are used to monitor the converter performance (their ability to store oxygen).

My advice is to visit a dealer (OPC) parts department and find out what the current part number is for the correct sensors for your car. Whether you buy the sensors there or elsewhere is up to you.

If you install the sensors handle them with extreme care! Do not touch the tips. The sensors are extremely sensitive to static discharge. Do not test their resistance. In fact just remove one from the box and immediately install it.

Do not use any thread lube.

IIRC the ones I have replaced which I bought from a dealer parts department came with a thread lube already.

Remove the old ones with everything dead cold and install the new ones with everything dead cold.

[Matt Seabrook]

Don't want to throw a spanner in the works but both those codes could relate to a lean mixture. Some proper testing may be a good idea before throwing a couple of sensors on the car. If the sensors are old then it's not going to hurt the car by fitting some new ones but it may not fix your problem. What are the live readings you have for the sensors or do you only have codes?

[Lamplighterson]

Matt Seabrook said:

Don't want to throw a spanner in the works but both those codes could relate to a lean mixture. Some proper testing may be a good idea before throwing a couple of sensors on the car. If the sensors are old then it's not going to hurt the car by fitting some new ones but it may not fix your problem. What are the live readings you have for the sensors or do you only have codes?

Live codes show zero voltage if I recall. Will double check tomorrow. Thanks for the help/advice. Will report back.

[Richard Hamilton]

When I said sensor heater relay, I mean the relay on the panel under the carpet, behind the rear seats. One relay controls both sensors.

[Lamplighterson]

Richard Hamilton said:

When I said sensor heater relay, I mean the relay on the panel under the carpet, behind the rear seats. One relay controls both sensors.

Aha! Thank you so much. Will check that tomorrow too.

[Richard Hamilton]

By the way, what diagnostic tester are you using? If it is a generic OBD2 tester, then you may have other Porsche-specific codes that generic testers cannot display. The UK '98 996 isn't OBD2 compliant.

[Rockster]

Matt Seabrook said:

Don't want to throw a spanner in the works but both those codes could relate to a lean mixture. Some proper testing may be a good idea before throwing a couple of sensors on the car. If the sensors are old then it's not going to hurt the car by fitting some new ones but it may not fix your problem. What are the live readings you have for the sensors or do you only have codes?

My references state that P1115/P1119 faults may be stored but these are not the actual faults.

I see nothing that indicates any the P0131/P0151 faults can arise due to a lean mixture.

The fault is logged when the voltage level of the sensor is below 150mV.

There are several tests.

1) Disconnect all sensors. Turn ignition switch to on. Using a digital voltmeter check the voltage reading between pins 3 and 4 of the sensor connector. Read approx. 450mV. Connect digital voltmeter at sleeve pin 3 and ground. Measure approx. 720mV.

NOTE: Do not use the ECM ground terminals when measuring input/output voltage or signals. Use a ground other than the ECM terminals, such as battery ground.


2) Remove sensor connector and connect a digital ohmmeter to pin 3 and the oxygen sensor housing. Read infinity. Connect digital ohmmeter to pins 3 and 2. Read infinity. Connect digital ohmmeter to pin 4 and the sensor housing. Read infinity. Connect digital ohmmeter to pin 4 and 2. Read infinity.

If no short to ground can be found the ECM may need to be replaced.

[Lamplighterson]

Rockster said:

My references state that P1115/P1119 faults may be stored but these are not the actual faults.

I see nothing that indicates any the P0131/P0151 faults can arise due to a lean mixture.

The fault is logged when the voltage level of the sensor is below 150mV.

There are several tests.

1) Disconnect all sensors. Turn ignition switch to on. Using a digital voltmeter check the voltage reading between pins 3 and 4 of the sensor connector. Read approx. 450mV. Connect digital voltmeter at sleeve pin 3 and ground. Measure approx. 720mV.

NOTE: Do not use the ECM ground terminals when measuring input/output voltage or signals. Use a ground other than the ECM terminals, such as battery ground.


2) Remove sensor connector and connect a digital ohmmeter to pin 3 and the oxygen sensor housing. Read infinity. Connect digital ohmmeter to pins 3 and 2. Read infinity. Connect digital ohmmeter to pin 4 and the sensor housing. Read infinity. Connect digital ohmmeter to pin 4 and 2. Read infinity.

If no short to ground can be found the ECM may need to be replaced.

Most appreciated. Before I go further I took some actual values today via software.

Bank 1 o2 sensor reports -0.41V
Bank 2 o2 sensor reports -0.41V

with PO131 and PO151 fault codes present.

Unplugging bank 2's o2 sensor (which had been a replacement bosch part 2 years ago) results in:

Bank 1 o2 sensor reports +0.15V (variable moving up and down)
Bank 2 o2 sensor reports +0.43V (but this is disconnected)

with no fault codes appearing after reset.

So effectively when bank 2's o2 sensor is removed, bank 1's sensor works.

Thoughts?

Edited by Lamplighterson on Tuesday 23 April 14:17

[Rockster]

Lamplighterson said:

Most appreciated. Before I go further I took some actual values today via software.

Bank 1 o2 sensor reports -0.41V
Bank 2 o2 sensor reports -0.41V

with PO131 and PO151 fault codes present.

Unplugging bank 2's o2 sensor (which had been a replacement bosch part 2 years ago) results in:

Bank 1 o2 sensor reports +0.15V (variable moving up and down)
Bank 2 o2 sensor reports +0.43V (but this is disconnected)

with no fault codes appearing after reset.

So effectively when bank 2's o2 sensor is removed, bank 1's sensor works.

Thoughts?

Edited by Lamplighterson on Tuesday 23 April 14:17

Bank 1 sensor's 0.15 volt reading is not a sign the sensor is working. It is a sign it is not working.

For a healthy up to temp sensor its voltage level should vary with a low of under 0.1 volt to a high of over 0.7 volt. At idle the period is on the order a once a second maybe longer out to nearly 2 seconds.

No fault codes just means the DME didn't have the time to perform a full check of the sensor operation. After the codes are cleared this can take some time (driving time). A few moments of running at idle is not nearly enough time.

Some OBD2 code readers/data loggers support/provide access to some of the OBD2 sensor tests. These tests -- if supported by the code reader -- can be executed and can likely highlight the bad sensor or sensors.

I note with both codes present this can be due to a common fault and this could be the wiring harness or even the ECM and its "final stage". (I also note in both cases only one fault code could be real, the other fault a false alarm. The trick to state the obvious is to figure out which is which.)

Anyhow, a suspected bad ECM or even a bad wiring harness is not to be trifled with. One wants to be sure these are both ok and also during their checkout not harm (static discharge) into the harness or the ECM.

Even if they both prove to be ok there's the risk the bad sensor could damage if not the harness the ECM.

[Lamplighterson]

I see. It's just that plugging the sensor back in immediately resulted in a fault code. Once cleared and disconnected the code did not return at all - even with the car running for some time...

Plug it back in and up it pops - within 10 seconds.