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I have a couple codes and also have a whistling-like noise that I think may be due to broken bits of honeycomb from the precats (see the image from oxygen sensor hole and scope). I think the cats are blocked up and one is obviously damaged as well. The first question is if this just normal use or if something is causing this (and hence replacement may be waste). The other question is what is the quality like for the eBay versions? They are only $150 while Porshce wants over a gand for it. Here are the codes btw:
P0421: Catalyst System Efficiency Below Threshold Bank 1 – Upper limit value exceeded, test conditions are not completed, fault is currently active and causing a DTC light.
P0431: Catalyst System Efficiency Below Threshold Bank 2 – Upper limit value exceeded, test conditions are not completed, fault is currently active and causing a DTC light.
I had a P0421 for years while getting the CEL light, the issue was a completely gutted Catalytic convertor. There was literally nothing left in the cat when the dealer finally pulled it out, only one was impacted.
Ideally, they should last forever. A number of things can get in the way, failing o2 sensor the most common.
The Porsche dealer I bought my 08 GTS from had put in an aftermarket cat and an equal quality new o2 sensor. While the o2 sensor failed less than a year later the cat has been fine for the past 35k+ miles.
And yes, the Porsche dealer selling the used car outsourced the repair to an indy shop using aftermarket parts instead of using their own Porsche shop and parts. Happens all the time, apparently. I only discovered this when I pressed them for an invoice to prove the work.
My initial plan is try to some O2 spacers and see what happens. Then maybe replace the O2 sensors...I have one on hand maybe get the other one for the bank and see what happens. Of course, I am a bit concerned about destroying the O2 sensors if the cat isn't working right...is that possible?
BTW, for the O2 spacers, you just put them on the downstream side right?
Last edited by AeroStress; Mar 21, 2025 at 01:46 PM.
Hey...is it possible to just remove the honeycomb (it looks blocked up and I think its causing too much back pressure and affecting gas mileage)? I am in a state with no inspection. Maybe I could at least try that first? Not sure how long it takes to remove and reinstall in though...if too long maybe that is a bad idea.
I thought if the state did not have an emissions check, that it wouldn't matter, but it turns out it may be illegal. I didn't know this. But it seems the felony aspect is reserved for selling/trade of cats (illegal possession or part of that loop)..and that makes a lot of sense.
Its a weird law. In a state with no emissions, no one cares if you have a check engine light due to bad cats. That is totally legal, yet its still bad for the environment. Bypassing the checks would have no net effect on the environment, which is why I couldn't have fathomed that would be illegal (but it is due to an inconsistent law between the federal and state levels). It seems maybe just leaving it alone is a reasonable option.
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PreCat Issues - With Images
