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Questions on Cat affects on pre O2 codes

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I have a 1999 - 996C2 'Tip' with 160K (kms)

Two weeks ago the car's idle went south - really ranging all over and the CEL came on.

Codes were P1317 (Misfire cly 5), P130, (Bank 1 before Cat O2 range), P150 (Band 2 before cat, O2 range), P1124, (Bank 1 O2 Sensing) and P126 (Band 2 O2 sensing)

So my mechanic pulled the number 5 plug and noticed it was oiled up and that the coils were origanal to the car. The thoughts were that the coil in #5 went bad causing the misfire which generated the other codes.


Six new plugs - six new coils installed. Codes cleared.

Within 20kms driving away from the shop the CEL comes back on. I read the codes when I get home and phone my mechanic. With the exception of the misfire code - the other four codes all relating to the pre cat o2's are back.

Next step was changing the pre 02 sensors. The post sensors are six months old. He cleared the codes and within 50kms the same codes were back on.

The idle is rough and hunting around between 700-800rpm once warmed up and stopped at a light. I notice that foot on the brake/foot off the brake makes a difference in the idle. Foot on the brake it drops as low as 500rpm. Take your foot off the brake and it rises to 700rpm in drive (Tip).

The car warms up with a lot of black carbon even spitting a bit of it on the garage floor with a bit of water and the tailpipes are coated with carbon like never before. Looks like it is running rich? There is NO smoke at anytime either before it comes up to temp or after, or if you hit the throttle hard. There does not seem to be a loss of power - throttle pick up is still crisp - although the engine note above 3000rpm seems 'throatier'.

MAF was two years old but we swapped it with one from a car that was running fine. It made no difference and all four codes came back.

If you run this car in this condition - the CEL will come on inside of 100kms.

The #5 plug was pulled again just to check on it. I had a great fear that I had either a ring issue or a cly wall score issue. The plug didn't seem to bad but it only has 200km's on it.

The only thing my mechanic is now suggesting is that I have a bad cat. Now there is a bit of a rattle out of one (passenger bank) but my question is can a bad cat cause these codes, idle issue and black carbon issue? I know a misfire dumping fuel in can cause a bad cat but I never had any flashing CEL's. Someone suggested that the cat could be plugged up causing an issue but why do both bank pre cat sensors come on and why both banks with equal carbon?

I can't pass a clean air test in this condition as even without a CEL - my area will not pass a car with these codes in memory even without a CEL on.

About ready to tear my hair out. So what if I replace both cat's and it doesn't fix the issue - that's my big worry.

I'm temped to install a new MAF just to eliminate that area - as his used one *could* have been an issue. The MAF even at $130 - it's cheaper then a Fabspeed 200 CEL sport cat (which is the option I'm thinking about as factory cats are $2K EACH!)

I just don't know why a bad cat would affect the motronic into a fuel rich state. Is it possible?

Edited by torontoworker
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At this juncture, I'm not convinced it is a bad cat. You need to get the car scanned with a Porsche specific system (Durametric, PST II, PIWIS) looking specifically at the pre and post cat sensors while the car is running. These systems can graphically display the sensor voltages in real time, which will indicate how well the cat is doing:

Good cat: ThreewaycatandO2sensoragingdiagnostics_z

Bad cat: ThreewaycatandO2sensoragingdiagnostics2_

While hooked up, you should also have the car's fuel trims checked.

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He was looking at the O2's using Durametric (pro ver with a new update) and the real time numbers looked fine with car running. Fuel trim?? Affecting both banks? I've got no codes for injector issues, no codes such as 420/430 (Cat's).

I currently have 1124/1126 in memory (Rich Threshold) So far... No CEL - yet. Although it's been under 50km's since a code clear.

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If he is using the Durametric, he should be able to generate O2 sensor curves like the ones I posted above to see how the cat is doing. Take just a few seconds to collect the data.

It also look like your DME is trying to richen up the mixture, which is why I suggested looking at the fuel trims.

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I think in the morning I'm going to bring my laptop out to the car - plug into the OB2, place it on the passenger seat and record the fuel trim numbers while I drive (I have Amateur Durametric for the 5.2.2). Any other settings I should select? I think there is a limited number of modes I can record and store to an excel sheet at a time.

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I agree with JEP that its probably not the cat. Before you start throwing a bunch of parts at the issue you need someone that has durametric, PST2, Piwis and knows how to use it correctly. Also there are two TSBs you should know about from Porsche.

  1. Oxygen Sensor Connections


2. Oxygen Sensor Heater Faults: Update of the DME Fault Software (ME 7.2)


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Update: Success! A lession in Murphy's Law. It was a bad MAF. But... the indy who had swapped out my MAF for another and tried this newish part from another car he had in that day - put in a MAF from an egas car - not a MAF from a 'cable gas' car. No wonder there was no improvement. It took another shop to figure this out. The part numbers are different and the operation on the Motronic is important. Clean air test went perfect after. No new cat's needed. Just like doctors - second opinions are always good! Thats for your suggestions here - it helped me to decide to try another shop.

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