I was heading out for a drive this morning when my CEL came on. I decided to turn around and head home to play it safe. When I got home, I scanned my car with my iCarsoft and received the following error code: P0420 - Catalyst System Efficiency Below Threshold - Bank 1. There are no noticeable symptoms, as my car seems to be running and idling smoothly with normal power and fuel consumption. After doing a search here and elsewhere

For some context, I have an '07 C2S with factory exhaust with the exception of the "Fister Mod" mufflers from FD Motorsports. I replaced the spark plugs and coils in '19 shortly after I purchased the car with 50k miles on it at the time. I currently have 63k miles on my car. I don't have all of the service records from the previous owner, but I have no indication that he replaced any exhaust components or any other emissions related repairs. I only use top-tier fuel (usually Shell) and run a bottle of fuel system treatment through it periodically. The first owner lived in Las Vegas, and I live in Denver and don't drive the car during the winter when there is salt on the roads, so my car if virtually corrosion free. It's also worth noting that I am using a high Moly/anti-friction oil (Millers CFS 10W50 NT+) which some will say can damage cats or O2 sensors, but I haven't read any quantifiable accounts of people here having oil related cat issues running Millers, Driven, or other non-A40 approved oils. Regardless, it's a data point I wanted to share.

One post I read on another forum stated that the cats internal elements may have failed or broken apart. I gave a gentle tap to each cat with a rubber mallet and din't hear any loose or broken parts rattling around in there. I'm not even sure that is a reliable method for diagnosing the internal components of a cat, but it was worth a shot and nothing seemed out of the ordinary.

My question for those who have had this issue or know how to troubleshoot it is, what is the most time efficient and cost effective way to diagnose the root problem? Should O2 sensors be replaced at a certain time or mileage interval on these cars? I am wondering if I should just go ahead and replace the O2 sensors, but I don't want to just start throwing money and parts at a problem when I don't know what is causing it. It's hard for me to believe that the cat has actually failed with only 63k miles on it, but maybe it is more common than I think? I'm really hoping that's not the case here.

 

Running a high ZDDP oil will not damage your cats or O2 sensors on a healthy engine.

My 996.2 had this same issue a long time ago. I also gently tapped on the cats with a rubber mallet and didn't hear anything but after I got the cats off I could shake it and hear noise and could see the honeycomb was broken inside. I attribute this to the high vibration experienced in the normal life of the cat. After replacing the cat issue solved. You can use Durametric to monitor your O2 sensors; post cat voltage should stay constant around ~ .7V. If the post cat "follows" the pre cat then the catalyst isn't working:


New cats are expensive. I took my broken ones and sold them to a metal scrap yard (got $150-200 or so if I recall?) and then found used cats from another forum member. Normally I wouldn't buy used parts but in this case it was an excellent solution and there was little risk. I wouldn't put a used AOS on my engine, but a used OE cat is fine. You can also go aftermarket cats and other stuff but I preferred not to go that route (street car and don't want to have to mess with O2 spacers and all that nonsense).

 

silver_tt, the signal after the cat should be close to zero with a functional cat, the signal in front of the cat between 0,2V (lean) and 0,8V (rich mixture). Your pictures show cats that are toast. Is that correct?

 

Correct -- my images are of bank 1 and bank 2 cats that are both bad before the repair. Port injected engines don't control the fuel trims nearly as tightly as DFI engines do so you will see the pre cat oscillate between 0.2-0.8v once a second .. rich/lean, rich/lean, rich/lean, back and forth, back and forth..... but the post cat should be pretty close to a flatline if it's working correctly. Post cat sensors should read about 0.7v almost steady. Certainly it should not be "following" the precat like my images show.

Monitoring of the 4 O2 sensors is a good way to test to see if the cats are shot because unfortunately I also experienced that hitting it with a rubber mallet and other observations didn't reveal that the cat was bad.

Here is what a good cat reading should look like. Oscillating solid line = pre cat, dotted line = post cat

 

Just clear the code. Make not come back for a long long time.

 

OK, thanks for the info. My engine does burn a bit of oil (roughly half a quart every 600-700 miles), but that has been consistent since I've owned the car for over 3 years and 13k miles, so I would consider it a "healthy" engine.

Do you need a Durametric to run the O2 voltage test, or can other diagnostic/scan tools perform this test, like an iCarsoft or Foxwell? It sounds like this is the only way I can be certain that it's a cat or not. Can a bad O2 sensor with fluctuating voltage or readings potentially be the culprit? I still think it's odd that a car with only 63k miles would have a bad cat, but I guess anything is possible. Based on the secondary O2 sensor location, I would think the cat would need to come out regardless to replace the sensor, which would give you a chance to have a look inside the cat.

I agree regarding the aftermarket options, spacers, etc. Seems like there are a lot of band-aid fixes out there like spacers, but it just ignores the problem. I wouldn't rule out aftermarket cats, but my car is a street car and I'd prefer to keep it mostly factory/OE. If I were to need a new cat, I would likely look for used ones like you did.

Yep, this is what I did for the short-term fix. I just want to make sure I have a plan when/if it comes back on.

 

I'm not sure which other tools can monitor the O2 voltages but I know Durametric can for sure because I used that. I replaced my O2 sensors first (cats actually do not have to come off but it's very tight and many swear words were said in my garage that morning) hoping it would fix it just as you are thinking but alas it didn't solve anything. I also cleared the code numerous times which may work in the meantime if the cat is marginal as opposed to just flat out bad -- but in the long run they probably have to be replaced based on my experience. Mine was a little higher mileage than 63k but not much, I think around 80k when I had to do it. The forum member I bought the cats from was a great guy and they were everything he represented them to be -- low mileage and in excellent shape. After swapping them in it completely solved the problem and when I went back and did the same test again, it looked like the graph I posted in post #4 instead of the post cat "following" the pre cat like shown in post #2. Do note that you may have to drive a while (like 100 miles) before the emissions reediness states are set for emissions (it is not instantaneous).

 

Sounds good. Thanks again. Just curious, when you were clearing the code, how long did it take for it to come back? I'm not showing any active or pending codes after clearing it, but then again, I only went for a short drive afterwards before re-scanning my car. I'm going to drive it a couple of more times over the next couple of days and re-scan.

I did a quick search for used cats and unfortunately it's slim pick'ns. There was a set here for sale last Fall it looks like, and only one current listing of a set on eBay with unknown mileage and an asking price of north of $1k, which seems extremely expensive considering the unknown condition and mileage. Just curious, but how long did you have to look before you found a set or the one you swapped out. I might have to bite the bullet and go with a new one, or spend a little more and go with Soul or Fabspeed aftermarket cats. That's definitely not my first choice, but it seems my options are limited.

 

I don't recall how fast the code came back because it was like 10 years ago (damn it I'm getting old haha). But probably the code would come back within a week or so of daily driving.

I got very lucky that I found a matching set quickly. Right around the time it happened a guy posted that he was parting out the car and it worked out really well because the cats were low mileage and exactly what I needed. I paid him with a credit card through Paypal for protection in case it was a scam or misrepresented but sure enough he mailed them right away and they were everything he said they would be. You could try to call someone like LA Dismantlers who specializes in Porsche. I bought a alcantara headliner from them for like $150 (you can negotiate down with them) that Porsche sells for over $2,000 new after someone put a golf club through mine and the used one they sent me was literally as good as new. If you could find cats fairly low mileage that was a front-end or side wreck or so you would be in business. Installing them is pretty easy too. A lot easier with a lift but no big deal. If I couldn't find anything or decided to go with new OE cats I would probably would have just bought them them from Sunset Porsche since their prices are very good and I have had a good experience buying parts from them for a long time for my Porsches and Audis.

 

Replace o2 sensors first (might as well do them all)... if the problem persists you can:
Replace the cats (most likely)
Add the O2 sensor spacer/mini cat (I wouldnt do this with a stock system though, its mostly for aftermarket cat/xpipe systems)

 

GIG,
Where are you located? Maybe a Rennlister nearby will loan you his Durametric.

I have Soul 200 cell cats on my 2008 C4S and love them. The O2 sensors screw right on and function properly. Plus these cats provide a very nice upgrade in exhaust notes with my Fister muffler cans. Just an option for you to consider.

Scott

 

Great minds think alike. I went over to a friend's and fellow Rennlister's house last night, and we used his Foxwell to look at the O2 sensor readings in more detail.

After comparing bank 2, sensor 2 (the bank that did not have the code) to bank 1, sensor 2, the voltage readings appeared to be almost identical between the two banks. I forget the exact number, but they both seemed to hover at about .7V until you rev the engine, in which case the voltage would drop momentarily before returning back to ~.7V at idle. Both banks exhibited the same behavior. I was looking for the spikes that @silver_tt mentioned and depicted in his graphs, but we didn't see that. I forgot to look at the graphs that show the sensor 1 voltage, but I can only assume they were performing normally.

We also performed the low-tech cat check by measuring the temperature of the cat using an infrared thermal heat gun. I've learned that a properly working cat's outlet temp should be roughly 100 degrees Fahrenheit higher than on the inlet side. We had already shut my car off and it had been sitting for a few minutes, but we decided to try anyway. Even with the car off (but having been up to temp), both bank 1 and bank 2 outlet temps were about 75 degrees hotter than the inlet side.

Lastly, we checked for stored and pending codes with the Foxwell, and I did again with my iCarsoft when I got home last night. Nothing is showing in stored or pending, so I guess I will just chalk this up to a fluke for now. I'll scan for pending codes periodically over the next couple of weeks, but I'm hopeful I'm good for now.

With all that said and having done some research, I would absolutely go the Soul Performance route when/if I have a cat give up on me. Used cats are hard to come by and are a great unknown, even with low mileage, and a new factory cat almost costs what a set of Soul 200 cell performance cats cost. I already have the Fister's, and I'm sure the Soul cats would only make it sound better.