Symptoms of a bad catalytic converter?

For a few years my car has intermittently thrown a P0421 "catalyst warm up failure" code. usually if the car's been sitting for a while. and a spirited drive for half hour causes this code to not come back for 6 months or a year unless the car is not driven much again for a while. so I am guessing that heat burns off some clogging in the right-side cat. I wanted to confirm before spending $2,000 on a new exhaust header with cat so I plugged in my scanner tool and monitored the cat voltages. opinions? would 0.2V reflect a bad cat compared to the left one on B2?

Hey @Fan987 , it's smart to do your own diagnostics with the scanner before dropping that kind of cash on new headers. It’s interesting to see the voltage difference you captured for that P0421 code. What do the readings for both sensors look like when you hold a steady RPM or while you're driving? Adding that extra info might make it easier for others here to help you confirm a diagnosis.

I think we need to see the fuel trim for both sides with the O2 sensor voltage....

thanks for the reply. first photo is at idle. second is at 2K.

The fact that they're matching is a good thing. And the numbers are good. I might consider swapping the output side O2 sensors and see if the error code moves. If not - then the cat is suspect.

I replaced both bank1 O2 sensors recently and the P0421 still came up again. further evidence that the cat is failing. I appreciate your reply. I was hoping for a solution that costs less than $2,300 plus labor. maybe I can install it myself but 4 hours labor is reasonable. even with just mid mileage I guess cats occasionally clog. I wondered if the tune was compensating for a bad O2 sensor and that ruined the cat (during prior ownership). that very well could have happened. but whatever, I'll just replace it... my regular shop gave me an estimate and said there are only a few 987 headers left in the market. I'll be getting one of the last that shows up in any NOS inventory. so very soon we'll all be scrounging parts yards for cats.

Please do let us know the outcome. It helps close the knowledge loop that makes P9 so much better than Facebook..

I got a quote from my local favorite shop for the exhaust header/cat of $3,000. I'm tempted because that's 600 labor. the header just went on sale at one of the sites for 2000 but I would have to deal with the core charge and refund while my shop will do that for me. I'm not in a hurry to do this because the code is intermittent. doesn't come back since I posted this. it's a once a year kind of thing. I want to get it done so the O2 sensor isn't fouled again but it's not a priority. I'm trying to get a track car done and I just bought a 997 c2s this week.

I haven't gotten this done yet but it's still on my to-do list. I found this link to an aftermarket OEM style replacement part: MagnaFlow 5411931. the part is available from several locations including FCP Euro : https://www.fcpeuro.com/products/porsche-catalytic-converter-magnaflow-5411931 why I love this: no core return needed and the price is $1,944. I might be able to install it myself. but that raises another whole issue... which might be better to deal with in my own garage rather than at a shop under pressure to keep cars moving... my car doesn't have much rust underneath so maybe this won't happen but it's still a common problem with exhaust header studs... 😂🙈 if you know what this is then you know... 😬

I am facing the same situation and thought of a solution like this, but would it really only make sense if both sides are getting replaced to help match the power output should it now be a bit stronger off this bank of the engine? Just a thought...FYI, my situation I think it is due to a blown AOS unit even with a quick shutdown and a tow. The AOS failure seems to have resulted in a fouled Cat on the one side. Mine too was intermittent, and then has become pretty much a constant code even after replacing both in the Upstream & Downstream O2 sensors on that side.

you raised several points . first, I don't think the AOS would affect the cats or the O2 sensors . when it fails it just makes a mess around the intake and the car is pretty much not drivable. a bad cat just doesn't restrict the emissions. I spent some time learning about all of this. OEM cats start off with about 600 and slowly lose effectiveness. carb requirement is 400 so car makers bump it higher. whereas a 200 sport cat won't clear the smog check in States like CA. but an old worn out cat basically becomes a 200. the back pressure from one cat to the other is so trivial that it won't have a big impact on the engine or the tune. it's like 1% less restrictive than the other if they're mismatched. I wouldn't worry about that. I am planning on picking up that magnaflow which is equivalent to the OEM. I don't think you need two of them. but I could be mistaken. I welcome anyone else's opinion.

Have you tried a Catalytic Cleaner additive to the gas? I've had good luck with a German made product OXICAT, I believe it is called, that is safe for oxygen sensors, etc.. I've used the product in a 1998 Lexus LS400 with about 300K miles that thru a catalytic converter code....?? As it turned out, the spark plugs were burned down nearly to the porcelains on iridium plugs .... Since plug change the CEL has disappeared... huh!! They are great, but even iridium doesn't last forever. I've changed them once before.... and then I gave the car to a daughter....

Why is that PDF instead of a link? I don't want to use fuel additives.

The PDF is a pix of the receipt from an on-line purchase I made on Ebay some years ago.... I didn't have easy access to the link. The product works, but if you would rather purchase a new catalytic converter you should certainly pursue that path. Cats are expensive, and may or may not work as advertised, especially AFM cats; there are different standards that need to be met depending on the state requirements [some will, and some won't], on the annual emission tests, so aftermarket replacements, if found, aren't always effective depending on the testing protocol. Also, plan on replacing up & downstream sensors with a new cat , again, as warranties may require their replacement. I can't imagine what Porsche wants for "new" catalytic converters? I'd also replace your spark plugs [& coils?] unless you have just done that? Bad ignition can trigger catalytic converter error codes too. Just some thoughts, I try to take the cheaper and easier fixes first, as you getting ready to spend a fair amount of money?

Oh, nice, if I don't open your PDF, it's because I want to waste money on car parts. Buzz off, buddy.