2 separate CEL codes (during/after 4250 mile trip)...

Hi gang, Well, I've been home for two weeks as of this evening, but I have continued to recover from a terrible cold and I'm finally really just now getting back into the swing of things. The trip was just outstanding, and I appreciate everyone here on P9 who was so supportive of my journey. Stay tuned as I'm hoping to put something together outside of P9 via Food Biker / Road Feast. Before anything else and following up on the previous thread... after all the driving, I'm pretty convinced that the white smoke thing is physically tilt-level dependent per how the car is parked. If my 2.9L is parked flat or tipped backwards on a slight angle, I'll get the major smoke at start-up. Not too concerned about it. Especially as I was essentially parking in different spots/angles for a month, it was a great way to test it out. Given that some times had no smoke at all, I'm guessing it was because it may have been parked slightly downwards. Following up via the trip, I think at about 3K miles into the trip, I had thrown a CEL light, aka "Check Engine - Visit Workshop." 2 separate codes were present, and I blew the same two codes again at 4200 miles (just 50 miles and at my final hour home). I'm guessing that the fuel related one was what brought on the CEL and the other code was silently there. In all cases, I noticed absolutely no change in performance doing the entire trip - the car ran smoothly from what I remember. Here's what I saw when I pulled it all up on the PIWIS 2 ; let me know what you guys think -- especially given my upcoming plan of attack. 1) This code was: P0441 Evaporative emission system (below limit value) This is clearly an error that involves the EVAP system. I had seen the same exact code a few times earlier in the year (the last time was literally when the car reached 260K miles). Back then, I swapped out the EVAP Vapor Canister Purge Solenoid and attached hose, and the error never came back. Turns out that upon removing the original, I bench tested the original and it was just fine too. I replaced the gas cap, but that didn't do anything either. Now on this trip, I think it blew the code some point possibly after filling up the car with BP (non top-tier) gasoline earlier in the day. The second time that it happened, I accidentally and clearly overfilled the tank with Citgo (top tier) gasoline -- an event which rarely ever happens as I'm pretty careful in that regard. That light came unsurprisingly came on about 10 minutes after the fill up. So before I go in there and start tearing into the system, that one I'm probably just going to drive the hell out of it, use up whatever fuel that's in there, and see if it comes back. Hopefully the carbon canister hasn't been damaged from the overfill but who knows. 2) The second code was more concerning, which was P0431 Warm up catalyst efficiency below threshold bank 2 (below limit value) I could swear I saw this error code earlier in the year, but couldn't find any records that I actually had. As I said, I'm guessing that this one didn't cause the CEL, especially given the the CEL went on right after filling up the car. This error code gets a bit more complicated. First, immediately before the trip, I had changed out the oil. After always using Mobil 1 0W-40, I decided to switch over to Driven DT-40, which is 5W-40 racing oil. The ticking sound at idle immediately went away and the car drove incredibly smoothly the throughout the whole trip. Total oil consumption went down to just a mere pint for the entire 4250 miles (as opposed to about 1 quart I think over 5K miles) and the ticking sound at idle immediately disappeared after the pre-trip oil change. Apparently this type of oil, formally marketed as Joe Gibbs oil, is not supposed to foul one's catalytic converter. I did read something somewhere the DI-40 (for DFI cars) might cause CC issues, but I didn't see anything involving the DT-40 -- which I figured was safe. Now, having put so much mileage on the car it during the trip, if the catalytic converter was fouled, I would've figured that all the insane driving would have cleared everything out after all that time and regular driving. I'm concerned it is something about that catalytic converter on the port a.k.a. the driver side, which is bank 2. Obviously, I'm trying to figure out where it is where things are going wrong. I know there are two oxygen sensors, and this code seems to involve measurement via the second sensor. I could prophylactically swap out the 2nd oxygen sensor, but apparently that usually won't fix this code. So, upon my research, I'm guessing there might be a small exhaust leak (I've swapped out the exhausts a few times) potentially the triangular CC flange that connects to to the exhaust's over-pipe. As many of us who have changed their exhausts before (I'm now back to running stock), there are three pressed studs on the cat that are usually stripped. In trying to remove one, I superficially damaged (a few pits) on the flange face. Instead of seriously sanding it all down (which might have taken off too much metal), I smeared some very high-temp Permatex gasket material on the gasket mating surfaces to fill any pits, before tightening the clamps down with 3 new brass nuts and 3 bolts. I'm wondering if after all the driving and shaking on the NJ Turnpike, I wonder if that joint could be ever so slightly loose but the exhaust is quiet as it ever was. I could also check the bolts again on the other end of the cat (intake manifold) when I'm deep in there, but I doubt they've come loose and I also doubt I damaged that gasket. Anyhow, I have to pop the winter tires on this week -- but really don't feel like tearing everything apart unless I really have to. Part of me really wants to remove both cats, have the left flange sanded down by a metal shop to make it perfectly flat again, have the shop remove the 3 old studs on the passenger side cat, and then reinstall everything with new gaskets. However, I really want to carefully rule things out before dumping Cataclean into the fuel tank. That's a mouthful on all accounts. Would love your advice before proceeding and creating unnecessary work. Cheers and thanks.

Looks like there have been a reasonable number of views, but I'm guessing that the lack of repsponses may be a case of: (1) TL;DR, (2) something has indeed been broken behind the scenes with P9's "new threads list" so people haven't seen this thread - I've alerted the higher ups, (3) I'm in rarified atmosphere with these two codes, (4) all of the above, or (5) none of the above. LOL. FWIW, here's the update. Car sat for two weeks as I recovered from a very bad cold, so there was start up smoke but it wasn't outrageously bad. Short drive to warm up the car, and then put it up on the Quick Jack . Didn't really expect any new CEL, but it was good to see that wasn't the case. 1) Going to monitor the EVAP code over time. Just hoping it maybe has to do with either (1) bad fuel or (2) the overfilling incident. Going to stick with Shell/Mobil/etc. as much as I can. 2) Upon inspection, the connection looks nice and tight, but I tightened them down just a smidge. Upstream cat (which I doubt is the issue) is over $150, so I'm leaving that alone. Downstream cat - I quickly removed it with my 22mm wrench (nothing special required), and I carefully applied a small amount of copper anti-seize on the threads. It wasn't a bear to remove given I had been in there last year. As these are only $50 to replace, I said - what the heck - and sprayed a little MAF sensor cleaner on the tip (which was ashy white) and cleaned it off. From extensive reading, cleaning the O2 sensor is apparently not possible and there's a high likelihood that trying to clean it will screw it may screw it up permanently. My gut said the following: If people have screwed it up before, whatever solvent that gets in there may actually cause some sort of a change (aka, possibly cleaning it) MAF sensor cleaner s largely the same stuff one uses to clean motherboards - so much so that I use a can of such for both purposes. As the other recommendation that seemed to make sense was 99% isopropyl alcohol (also used to clean motherboards), this seemed safe. Carb cleaners, etc. seemed way to harsh and may leave residue (what we don't want) If it was white (ashy), clearly it's getting crazy hot and any residue would burn off anyhow Again, I wouldn't attempt this UNLESS I knew I might replace it. Anyhow, cleaning (if successful) is apparently temporary and I doubt that's the issue. However, changing out the downstream one would be my next step. Too much of a PITA to swap out with the other side given the wire routing to test out and see if they would induce the issue on the opposite bank. My guess? Both issues will eventually return, but only the EVAP system is going to trigger a CEL out of those two codes. Half tempted to try the Cataclean at some point, but going to be conservative for now. Stay tuned.

Reading your experiences with these codes, and the paths needed to track down the culprits responsible for, bolsters my inner voices telling me it’s near time to purchase a Foxwell or equivalent code reader. I’ve been lucky (or blessed) to have been free of CEL’s during my ownership but owning a Porsche makes one aware of how finicky they can be. Its on my list of things to do in 2026.

Experienced PO421, same issue bank 1. Started with Cataclean, what could it hurt at 72,000 miles. No change. Plugged in my Durametric, after luckily finding an article on just that. I was able to see that my post cat O2 sensor was failing. Instead of a nice smooth line, it was spiking, think analog vs digital. Replaced O2, thank you FCPEURO, solved, I thought. While I was now getting the correct live signal, O2 sensor working correctly, I would still get PO421. Is the culprit the cat? Everything was pointing in that direction. Enter, my hero CHEVRON TOP TIER. I had been using a combo of mostly Sams (apparently not Top Tier) and other Top tier fuels. It took a little over two tanks, issue gone. Hopes this helps.

@WTFChuck - Definitely some sort of reader is always helpful. Again, as we discussed -- I love the P2 but an aftermarket reader is often easier. I like being able to see exactly what the dealer would see, but that's just me and lots of guys are using the aftermarket readers. My P2 paid for itself when I got it - as I coded back in broken stuff when I first got the car. @GotBox - Your reply is a HUGE help, as P0421 is the same code as mine, but Bank 1! Thanks. A few questions: 1) Was it your downstream O2 sensor that was failing? Wondering if I should just replace it and be done with it. Would like to avoid the upstream sensor due to the cost. Unscrewing isn't the issue; it's the wire run apparently that's a PITA. ​ 2) How did you measure the spikes? Were you running a PIWIS while running the car? Haven't done that yet. I have a feeling that's not a bad idea, but I never gave that a shot.​ 3) Going to avoid the Cataclean for now and would put the $25 towards the $50-something O2 sensor. Apparently very mixed reviews. What do you think?​ ​ 4) When you got the P0421 code, did you also get a CEL? Or did just show up on your codes the next time you read it? Guessing that my CEL was from the EVAP and not the P0431 -- as it came on within 10 minutes of overfilling the tank.​ **** I'm ultimately thinking that over the 4250 miles, I'm guessing I got some less-than-great fuel in there during my trip -- despite the fact that most gas stations I frequented were/are Shell/Mobil/Sunoco (Top Tier). The overfill was Citgo (Top Tier), so hopefully even that will blow out within the month. Again, during the trip -- a few times I had to resort to BP at least once due to limited availability and I remember 25 years ago back in Ohio (where BP was prevalent) -- hearing I think from a dealer mechanic that BP fuel would routinely foul Saab turbos (I used to drive a 9000S in the '90s) -- and I've generally avoided it since (not difficult to do as I live in New England). BTW - One potentially and unrelated very cool note. Since I got the car years ago, the passenger window wouldn't always go down when closing/opening the door -- which has likely a door sensor problem. But by the end of the trip, this issue seemed to finally go away. From past evidence, the car was obviously a driver-only car when I got it -- as the passenger seat was mint while the driver's seat was trashed. Accordingly, I'm guessing that one of the passenger door sensors saw ridiculously light use and was likely stuck as non-functioning. Meanwhile, regularly opening and closing the door over nearly a month's travel may have finally resolved the issue. In fact, on the PIWIS, the door sensor was likely gone from what I remember -- which was a nice bonus.

@foodbiker - First, glad you are driving and enjoying your Pcar. On to questions: 1. Yes, downstream O2. Diagnose O2 operation before you replace. As with many codes, there could be multiple reasons. I believe in this case, failing O2 or catalytic converter can throw this code. I am not a technician, so there may be others. 2. I used my Durametric, looked at Actual Values while running. See photo, this is how it should look. Can't seem to find the photo of how it shouldn't look, however if it's spiking, that's bad. 3. Yes, okay to wait on additives. If O2 is okay maybe a cat issue. If it's a cat issue, you'll need to find root cause of failure. The above video may help. In hindsight I think Techtron could be the way to go if needed to clean cat. 4. Yes, thew code, cel and same message on dash. No other codes. Poor fuel quality seemed to have been my issue. All other values were good when looking at upstream O2 data. Even at top tier locations you can still run into fuel quality issues Funny how all the poor fuel went to one bank! LOL. Keep us updated on your progress. Hopefully you can sort it as easily as I did. Eyes up, look ahead

@foodbiker - If not already using, FCPEuro or Pelican Parts is the way to go for lifetime warranties on CONSUMABLES!! I wish I knew sooner!

This is all amazing and incredibly helpful info, @GotBox - Seriously. You rock. As of right now, the CEL is gone after I cleared the codes, but I strongly suspect both issues will eventually return. For various reasons, I haven't really driven the car that much since the trip. Meanwhile, I think the O2 monitoring is something I'm going to look into and will certainly watch that YouTube video -- the PIWIS should allow me to do so -- it's just very tricky per computer positioning and I can't do so while driving the car. Is your screenshot of the two values AFTER the O2 sensor replacement? Scratching my head about bad fuel going to one bank (in which case is the opposite side for you), which doesn't seem to make sense. But what might make sense is that one side was compromised (same gas, bad O2 sensor or a dirty cat on your Bank 1)? Oh, definitely - I'm a huge fan of FCPEuro's return policy. I think they were the first, but ECS Tuning and Pelican are trying to keep pace. Given they're about 150 miles from me down in CT, free shipping to MA ($50+) is wicked fast so I just generally go with them. Also, at just under 265K miles, I'm guessing that some $12 "High Mileage" Techron is in order -- can't hurt and certainly not expensive to give it a shot before Cataclean, etc. The other option is for "BG 44K" - which gets superb reviews although it's $25. Can't help but think that the car was never "cleaned" before. Sounds like the cat could have been potentially fouled during the trip, and might be saved if I act quickly. Found a really interesting review of Techron - I've heard about it before. Given that I was going up/down altitudes and really pushed the car hard on the trip, I wonder if this was part of the issue. Read on:

@foodbiker - Thank you, and yes, I do rock! See photo Yes, those are both downstream live values at idle. Watch the video, it is short and concise. My screen shot is on a very short time frame, as in a few seconds. In the video, time frame is minutes. mV frame up/down, time frame left/right. You'll see after watching above. You don't need to be at speed to see and diagnose up or downstream O2's. However, not a technician, so there may be instances for more rpm for upstream diagnosis? Once I determined bad O2, continued driving. However I always have my laptop and Durametric cable in case of any code. I can't stand that darn cel being on, and not knowing why. My issue appeared to be a combo of poor fuel and bad O2. Fuel may have fouled or damaged cat and or O2? Someone suggested removing the O2, soaking in gasoline overnight. Then take a small wire brush (I used stainless steel) to clean it. This did not work for me. The P0421 continued to show up about every other drive after replacing O2. Yes downstream O2 working correctly. Apparently the downstream values were not off enough to trigger cel and code every time. Catalytic efficiency was still the issue. So before springing for new cats, ouch, I went the Chevron route. While I credit Chevron fuel, fairly sure it was a combo effect with the Cataclean? Excellent MesaGuy article, thank you

@foodbiker - I found the article (not Pcar specific, but a MUST READ) I referenced earlier at OBD ADVISOR: How to Test O2 Sensor With Scanner (Live Data Guide) | OBD Advisor Learn how to read O2 sensor live data with an OBD2 scanner. See normal vs. bad voltage patterns, upstream vs. downstream behavior, and common O2 sensor codes. www.obdadvisor.com

Again, great info @GotBox and thanks for posting all that here. Will def check out that article! Dual downstream O2 values - Great. I'm assuming that both Bank 1 and Bank 2 downstream sensors should be the same, so that's good to keep an eye out for. Agreed - I'm not one to drive around with a CEL if I can help it! It bugs me as well, although surely it's not catastrophic. I took my P2 with me on my trip and cleared the codes partway through the trip. Chevron - Did you mean Chevron gasoline and not the Techron fuel treatment. Or both? No Chevron stations around me in New England, but plenty of Shell and Mobil -- which come with additives that clean the engine at ever fill-up. Top Tier gasoline does not. My guess is that if you replaced the O2 sensor and the CEL = kept returning, your cat was fouled. Given they're self cleaning (I guess like an electric oven on "clean mode"), it probably couldn't self-clean correctly and do its thing. I suspect that's the same thing that's going on here. I'm just worried that my switching to Driven DT-40 fouled the cat, but I can't be sure. Meanwhile, I noticed less oil consumption than with Mobil 1 0W-40. Apparently, O2 sensors can't really be cleaned per my previous posts - but I figured -- why not give it a shot with the downstream one and the MAF sensor cleaner. A small wire brush can't get inside what appears to be an external metal shield. Kind of like cleaning the outside of a propane grill to try and clean the grates. I certainly wouldn't try to clean the $150 upstream one unless I 100% knew it was toast. I think my next steps will be: 1) Watch videos (thanks!). ​ 2) Now that the winter tires are on (with O2 sensor cleaned and exhaust flange tightened), start with just driving the hell out of it post-trip, and go through the tank of Citgo that I put in there a few weeks ago. Check for codes with P2, even if no CEL.​ 3) If (or when) CEL returns, put the P2 on it and check for O2 downstream sensor values, replacing O2 sensor if necessary.​ 4) Put in fuel treatment cleaner (BG44K, Techron, maybe Cataclean) and drive the hell out of it some more.​ Will definitely report back. Thanks for the help on this again, @GotBox !!!

One of the few challenges I've experienced myself, glad to help. Referenced Chevron fuel. Sounds like a plan Thank you for the updates. Keep 'em coming.

Anything to report?