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Old 07-10-2013, 08:18 PM   #1
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Calling on 986 Forum Master Mechanics

I have an intermittent P0430 alarm that has been haunting me for at least four years. It hid for a little while last year but has come back to annoy me. Attached are pictures of O2 sensor voltage levels ahead of the catalytic converters (both banks) and O2 sensor voltage levels behind the catalytic converters. As you can see there are somewhat erratic O2 sensor voltage level changes behind the catalytic converters on bank 2. The voltage may stay consistent (similar to bank 1) for a while and then the voltage will drop a few times and smooth-out again. What can cause this? Wouldn’t the inconsistency of the voltage drops exonerate the catalytic converter? Can a vacuum leak cause this condition on one bank?? The O2 sensors are just over 2 years old, with ~35,000 on them... I’m thinking swapping the sensors or replacing the O2 sensor behind the catalytic converter on bank 2. Any other suggestions or advice would be greatly appreciated.





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Old 07-11-2013, 03:15 AM   #2
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When you get the sensors ahead and behind the converter drawing similar lines (one should be a sine like wave, the other relatively a straight line), which you appear to have, it is the sign of a dying cat. I'm away from the shop at the moment, but I can show you curves for good and bad converters, yours appears weak at best.
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Old 07-11-2013, 06:48 PM   #3
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When you get the sensors ahead and behind the converter drawing similar lines (one should be a sine like wave, the other relatively a straight line), which you appear to have, it is the sign of a dying cat. I'm away from the shop at the moment, but I can show you curves for good and bad converters, yours appears weak at best.
What! I need to replace a catalytic converter with only 246,000 miles on it?
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Old 07-11-2013, 07:28 PM   #4
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How did the cats perform on your last CA smog test?
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Old 07-11-2013, 07:51 PM   #5
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How did the cats perform on your last CA smog test?
Passed with flying colors two years ago... I have to get a smog this month for August registration.

Since I am not getting a CEL (the P0430 shows-up in memory... Sometimes weeks later after clearing) (and yes the CEL works) I will probably go ahead and take it to a Smog Shop and see what happens.
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Old 07-12-2013, 01:07 PM   #6
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I would be interested to see what a healthy o2 reading looks like as well so I can compare with mine
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Old 07-12-2013, 02:52 PM   #7
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OK, here ya go:

Good cat:


Bad cat:
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Old 07-14-2013, 01:55 PM   #8
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Thanks JFP.

I filled my tank with gas that has Techron, and added some injector cleaner, then drove it hard a few times (hit 7K RPMs a few times) that seem to help a little. Here is a snapshot of Bank 1 (no appearance of issues):


And here are a few snapshots of Bank 2:






It seems the catalytic converter on bank 2 is starting to go... Woe is me...
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Old 07-14-2013, 02:47 PM   #9
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Thanks JFP.

I filled my tank with gas that has Techron, and added some injector cleaner, then drove it hard a few times (hit 7K RPMs a few times) that seem to help a little. Here is a snapshot of Bank 1 (no appearance of issues):


And here are a few snapshots of Bank 2:






It seems the catalytic converter on bank 2 is starting to go... Woe is me...
Unfortunately, it would seem so; that last snapshot tells the story as the two sensors are trying to follow each other. Sorry.
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Old 07-15-2013, 01:12 PM   #10
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Please excuse my ignorance but how do you know it's the cat and not the o2 sensor itself? TIA
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Old 07-15-2013, 01:33 PM   #11
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I have a CAT for one side never used if you are interested.
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Old 07-15-2013, 04:10 PM   #12
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Please excuse my ignorance but how do you know it's the cat and not the o2 sensor itself? TIA
Yeah, I'm not done playing with this... I will probably swap the O2 sensors to see what happens.

BTW the Jägermobile just passed the California Smog Test this afternoon. The hydrocarbon (PPM) was 5 (max allowed is 100 idle, 130 at high RPM) (average for passing vehicle is 17) and the carbon monoxide was 0.00% on both measurements. The %CO2 measured was 14.06 and %O2 measured was 0.5. The Jägermobile is still a clean running machine.
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Old 07-15-2013, 07:06 PM   #13
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I have a CAT for one side never used if you are interested.
Thanks for the offer, is it a TWC for bank 2?
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Old 07-16-2013, 05:18 AM   #14
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I have a related situation and question. My last two CA smog tests were close to failing. The max HC at low rpm is 58ppm; both tests, one this year and another two years ago, showed my '98 low mileage Boxster at 57 ppm- too close for confortable.

This year I did the following as suggested on this forum: replaced spark plugs, ran two tanks with techron, and drove the car harder than normal. The car was hot upon delivery to the test station.

What might be the cause of the high HC measurements ? Is there a systematic approach to eliminating various causes ?
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Old 07-16-2013, 06:39 AM   #15
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I have a related situation and question. My last two CA smog tests were close to failing. The max HC at low rpm is 58ppm; both tests, one this year and another two years ago, showed my '98 low mileage Boxster at 57 ppm- too close for confortable.

This year I did the following as suggested on this forum: replaced spark plugs, ran two tanks with techron, and drove the car harder than normal. The car was hot upon delivery to the test station.

What might be the cause of the high HC measurements ? Is there a systematic approach to eliminating various causes ?
Several things possible; dying cats will definitely do it, as will leaking injectors, or aging O2 sensors. Get the car scanned on a Porsche specific system like the Durametric used above, it will allow you to check the functions in real time.
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Old 08-10-2013, 02:48 PM   #16
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Originally Posted by coolbreeze551 View Post
Please excuse my ignorance but how do you know it's the cat and not the o2 sensor itself? TIA
Update, I finally got around to swapping the O2 sensors behind the catalytic converters and no change, I am still seeing the same voltage drops on the O2 sensor on Bank 2... Darn it.
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Old 08-10-2013, 11:57 PM   #17
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I have Porsche Boxster 2.5 1998 and it has failed the emission test on to high Co. The car has been serviced including fuel filter, plugs, oil, air filter, oil filter and coil packs. In addition to this its had new Cats, new o2 senors and I have cleaned the Air flow mass sensor. There are no error codes coming up, have you any idea why the ecu is providing to much fuel so its pushing up the Co emissions? I have cleaned the air system and I have mixed the petrol with injector cleaner, could this cleaner affect the readings? Please can you help? I saw your post and have a similar problem but with the Co. In you search for you problem have you found anything similar to mine and if so can you help.
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Old 08-11-2013, 09:01 AM   #18
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I have Porsche Boxster 2.5 1998 and it has failed the emission test on to high Co. The car has been serviced including fuel filter, plugs, oil, air filter, oil filter and coil packs. In addition to this its had new Cats, new o2 senors and I have cleaned the Air flow mass sensor. There are no error codes coming up, have you any idea why the ecu is providing to much fuel so its pushing up the Co emissions? I have cleaned the air system and I have mixed the petrol with injector cleaner, could this cleaner affect the readings? Please can you help? I saw your post and have a similar problem but with the Co. In you search for you problem have you found anything similar to mine and if so can you help.
Thank you
I have never had an emissions level problem, just the CEL with P0430 code which seems to be a catalytic converter starting to fail. I have not come across a similar problem as yours, it might be a fuel injector issue or MAF sensor issue? Have you cleaned the Throttle Body? How is the idle and driving performance?
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Old 11-07-2013, 04:46 PM   #19
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OK, so I got a fourth mortgage and purchased the factory manifold catalytic converter for my car, I plan on installing it tomorrow. Any recommendations on removing the original bolts on the original catalytic converter would be greatly appreciated.

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Old 11-07-2013, 05:05 PM   #20
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Make a concoction of acetone and auto tranny fluid, one to one (I said tranny, heh, heh) and spray all fasteners while they are hot. Easy when loosening. If the fastener is stuck, try some taps with a hammer, then try tightening the fastener. I know it sounds crazy but sometimes tightening will break the fastener free.

I would set your torque wrench at twice the torque spec for the fastener and tighten, loosen the fastener until the torque wrench clics and if doesn't come loose, try some more solvent and taps. Take it easy, many horror stories of having to drill out and tap the manifold bolts.

If no luck, then a torch to heat up the fasteners may work.

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