Two codes P0421 and P0431, bad cat?

**Smitty2A4** · 08-27-2009 06:13 AM

I have had an intermittent MIL for a while now, it would come on for a month or so and then it would go out and stay off for an even longer time. The car ran fine and gas mileage was normal so I figured it could not be anything serious. Well then came time for good old MA State Inspection, here is the thread with the details about that...

http://www.audizine.com/forum/showthread.php?t=303822

I failed because the light had come back on and it was my last day to get inspected and I figured it would be better to just get the R for now then have an expired sticker. Well the next damn day after I got the R the light goes off and it has been off for almost two full months now and next Monday is that last day of this mouth, meaning the last day I have to get my sticker.

Anyway I do not want to get into that too much in this thread, but more figure out what those two codes mean, those are the codes I was given for being the cause of the MIL. Here is what they apparently mean..

Warm Up Catalyst; Bank 1: Efficiency Below threshold P0431
Warm Up Catalyst; Bank 2: Efficiency Below threshold P0431

I am assuming I have two because I have two cats, what is the most likely cause of these two codes? I have done a lot of searching on this and some people say it is because the cats are failing and this is common on the 3.0 and that they cost 2-3k to replace (I have over 80k miles). Well if that's the case I guess I am done with this car, or I am not going to be able to drive it for a LONG time before I can get something like that fixed, esp. when it needs a timing belt too. Some people say it is the O2 sensor, others the ECU having the cat efficiency thresholds being set too tight. I really have no idea at this point I just want to make sure I can pass my re-inspection test so I can at least have a car while I figure all this out, I obviously don't have the time or potentially money to get this resolved before Monday but I do plan to rent an Autozone code reader just to make sure nothing is still coming up and know that I will at least pass. Also I have read about this anti-fouler mod which messes with what type of reading the O2 sensor gets. Could that help me to pass?

If this really is the cat there is no aftermarket alternative? I remember with my Jeep I bought a $100 carsound high flow cat where as the stock was $500. Yeah sure I could go catless but then I am still going to fail emissions testing. I am really going though one of those periods where I hate this car and want to get rid of it right now, I know I prob wont but when it needs a timing belt ($1,500), possibly new cats ($2,000), new brakes (I have the parts already) and a bunch of other small shit that I will not even bother to mention it is hard to not think about just ditching it.

**3.0allwheelaudi** · 08-27-2009 06:16 AM

I get the same code from time to time and my cats are still good, nothing showing the signs of a bad cat on my car. i have the code cleared and just go about my day.

**98A4TQ** · 08-27-2009 07:14 AM

I sure if your getting the cat efficency below threshold code that the non fouler mod will work, It did for me. I used info from this thread.

http://www.audizine.com/forum/showth...nti-fouler+mod

**Smitty2A4** · 08-27-2009 11:12 AM

Originally Posted by 3.0allwheelaudi

I get the same code from time to time and my cats are still good, nothing showing the signs of a bad cat on my car. i have the code cleared and just go about my day.

Are you able to pass inspection OK once the light is off?

**Dillon** · 08-27-2009 08:44 PM

Some kid i know with an a4 put regular gas in his car and it went on and off.

What octain do you use?

And yes about 40 to 100 miles after the light goes off it will pass.

**Smitty2A4** · 08-28-2009 06:06 AM

Originally Posted by Dillon

Some kid i know with an a4 put regular gas in his car and it went on and off.

What octain do you use?

And yes about 40 to 100 miles after the light goes off it will pass.

I have heard using shitty gas will cause it too, but I always use 93, I have never put anything less in the car and same with the previous owner which I know because he is my Uncle and loves cars haha. I have gone well over 100 miles with it off so I guess I am going to have it re-inspected either today or on Monday and hope for the best!

**Dillon** · 08-28-2009 10:50 AM

Originally Posted by Smitty2A4

I have heard using shitty gas will cause it too, but I always use 93, I have never put anything less in the car and same with the previous owner which I know because he is my Uncle and loves cars haha. I have gone well over 100 miles with it off so I guess I am going to have it re-inspected either today or on Monday and hope for the best!

I use 91 and every now and agian when i fill up i get the bank 1 code. I guess it could just be shitty gas or our cats are going out.

GO GET YOUR CAR INSPECTED BEFORE IT COMES BACK ON hahaha

**Smitty2A4** · 08-28-2009 12:48 PM

Alright I am gonna go get it re-inspected right now as I just realized that tomorrow will be the 60th day not Monday, so this is pretty much my last chance and I really have nothing to loose. I have faith that I will pass, wish me luck haha! I will report back what happens.

**road_tripper** · 08-28-2009 01:36 PM

google "obd ii drive cycle"; if you've recently turned off the mil light, you need to go through the drive cycle to pass emissions.

**Smitty2A4** · 08-28-2009 01:38 PM

I just got back and...

SHE PASSED!!!

God damn is that a relief, now I can not worry about that and focus on getting my new rotors and coilovers installed! If the code comes back I still want to have it looked into by a mechanic because I do not want to be in the same boat next year but honestly I do not think it is my cats, because I would think my gas millage would be sucky and I might have other codes to go along with it as well.

**Dan1969** · 08-28-2009 03:36 PM

So that means that your problem will probably still be there. That no-foul mod looks interesting. For info, the Bank 2 and 1 could be the after O2 sensors, since these report the cat emmissions. Glad it passed though.

**pradheep88** · 02-19-2010 08:50 AM

i got the same problem on my 3.0....i dont know wat 2 do...recently my check engine light was flashing and when i check it was miss firing so i changed my coil packs and spark plugs after doing dat the check engine light went off it didnt show p0431 p0421 after some time agian it came back...i have no idea wat 2 do for this car..my cats r good...please tel me a solution .......i went to a audi meachinic in queens ny he said he is gonna install o2 spacer and get rid of it....does it really work????coz he was asking me 400$ do dat????

**ocod** · 02-19-2010 09:26 AM

Does the 8yr or 80K miles emission warranty cover you? If so have dealer record CEL clear it and if it comes back the cats should be replaced????

**old guy** · 02-19-2010 10:50 AM

Originally Posted by ocod

Does the 8yr or 80K miles emission warranty cover you? If so have dealer record CEL clear it and if it comes back the cats should be replaced????

That is correct. The catalytic converter is covered under a federally mandated 8yr/80k.

**Mark** · 02-19-2010 02:05 PM

Sounds like your O2 sensor(s) heating element(s) are shot. However the codes are for both catalytic converters . . . to have all go at the same time would be kinda odd. Never know though. Test the impedance across the oxygen sensors to see what kind of resistance you're getting. I can't remember the correct range off the top of my head, my Bentley manual does that for me.

**dpatel** · 02-20-2010 07:50 AM

Can you check the impedance using VAG-com? If so, how?

Thanks in advance.

**betyouaint** · 05-16-2010 06:23 PM

Think I'm in the same boat on this one. The comments about using shitty gas caught my interest as I've just started experimenting with using lower octanes...

**GP2001** · 05-25-2010 05:18 PM

Hey guys, think I have the same issue. Just bought my 2002 Audi A4 3.0 Quattro with 120k miles two days ago, and apparently the guy I bought it from cleared the CEL before I purchased it, gotta love people! So I find myself with a CEL throwing out the notorious P0421 and P0431. Is there anyway to find out whether or not it is 100% just an ECM problem? I would go ahead an gut them and do the fowler mod, but I am concerned that my cats could go bad and I would have no idea (besides the obvious signs). Does VAGCOM allow you to clear CEL for gutted cats?

What is the recommended route to take when trying to fix this issue? I see that rockauto has the cats for $330 each, but I don't really feel like spending $700 on cats for this car after just buying it :(. I am thinking that I will just gut the cats and do the fowler, but I don't want to do that if the cats are fine...

**JohnTC** · 06-08-2010 01:44 PM

I got the code p0421 on my 3.0. that is the passenger side. I went to autozone and bought the defouler. I drilled a 1/2 hole, but my O2 is closer to 5/8 and won't fit. I need to get a bigger bit. I am also puzzled by my gas mileage. I have only been getting around 13 mpg around town. What would be causing that? I just changed all of the plugs and cleaned the MAF with no improvement.

**imnuts** · 06-08-2010 02:54 PM

Originally Posted by JohnTC

I got the code p0421 on my 3.0. that is the passenger side. I went to autozone and bought the defouler. I drilled a 1/2 hole, but my O2 is closer to 5/8 and won't fit. I need to get a bigger bit. I am also puzzled by my gas mileage. I have only been getting around 13 mpg around town. What would be causing that? I just changed all of the plugs and cleaned the MAF with no improvement.

If you're getting poor fuel economy, that would make me think that you have more of an issue with the O2 sensor rather than the cat, unless the ceramic catalyst collapsed/broke and is now adding a significant amount of backpressure to the exhaust. If the cat is broke though, I'd think you'd have more issues than just poor fuel economy.

**JohnTC** · 06-08-2010 04:08 PM

wouldn't there be a code for the O2 sensor? I'm questioning the cat myself, but it's the only code that comes up. Is there a way to check the O2? I remember reading somewhere about checking the resistance with a multimeter, but I searched again and can't find it.

**wurzburg** · 03-25-2013 08:18 PM

I had the same problem on my 3.0L: P0421 on and off for two years and gradually occuring more frequently. Less often also saw P0431. The dealer wanted to replace the cats for big $. I got lucky and managed to pass inspection on one of the better days. Then one day it got worse. The engine started and idled poorly and hesitated on acceleration - so I checked and found eleven codes: rich, lean, misfires and MAF. This was a blessing in disguise.

For $15, I purchased a can of special purpose mass airflow sensor cleaner and a set of tamper-proof torx bits necessary to remove it. I removed the MAF sensor and found that after 80k miles it was quite dirty. There was quite a bit of visible fuzz on the wires. Also, some fairly large leaf particles had made it through the air filter and were stuck on the MAF's inlet screen. My immediate gut feel was that the dirty wires and leaves were certainly enough to create some strange readings. I cleaned it all up, put it back together and cleared the codes. For several weeks now the car has been running like new and has thrown no codes.

I'm no expert, but his is the way I understand it: The catalytic converter has two oxygen sensors (front and back ends). These are part of a closed loop system to manage fuel flow. Ideally the engine is running at the stoichiometric point where fuel and oxygen exactly match. It can't really achieve this so the system does the next best thing. It flips back and forth between a tiny bit lean and a tiny bit rich. The front oxygen sensor chatters on and off in the presence of oxygen so the computer knows if it is above or below the stoichiometric point and adjusts the set points accordingly. When rich, there is excess fuel in the exhaust and the catalytic converter uses stored oxygen to burn it and reduce hydrocarbon emissions. When the mix is lean, the excess oxygen is stored back up again. This dithering within limits is the normal operation. The second (downstream) oxygen sensor is not supposed to behave like the first one because everything is supposed to be used up by that point. If it does chatter, the assumption is that the catalytic converter must be bad (or missing), because it isn't doing anything. Thus the P0421/P0431 codes.

An increasingly dirty MAF is consistent with the degradation in performance and greater frequency of MIL codes over time. I think what was happening is that the MAF measurement got so far off that the control module struggled to compensate. The engine probably sat on the lean side of stoichiometric, so the oxygen sensor thought the converter wasn't working and tripped the fault. Had it been running rich for an extended period, combustion of the excess fuel would have overheated the catalytic converter and destroyed it. So if you only have a dirty MAF, your cats are probably okay.

The moral is try cleaning your MAF for $15 before spending 100 times that on new cats. It's cheap, easy and worth a try.

**SlickFix** · 03-26-2013 08:58 AM

^^ Aye aye, Skipper. I'll try almost anything at this point.

**wurzburg** · 04-16-2013 05:48 PM

After a month and 500 miles of being code free, the P0421 and P0431 came back on. Cleaning the MAF sensor improved my engine's performance and fuel economy considerably and that continues to hold true. But it appears that I still have an on-again-off-again cat problem. Any ideas??

**mcanedo92588** · 12-28-2014 07:57 PM

Sorry to bring this back from the dead, but I am having the same issues as everyone else now. Plus the p0171 and p0174 "system too lean" issues at the same time. Man I really just need to be rid of this car, 7k in repairs in the 2 years that I have owned it, fml.