I Have a 1998 A4 Quattro. A week ago i had a CEL that said P040 something about catalytic not reaching lower threshold or something. it wasnt affecting my performance car running perfect and everything. I was going to take it to my friend that has a VAG-COM and check it out on Friday but this morning the car was idling SUPER rough AND black smoke was coming out of the exhaust. Trying to give it some gas it bogs down and doesnt ever climb over 2k RPM. Please help i need this car for the weekend i have an important meeting and its 4 hours away.

PS i am in the process of getting new O2 sensors for before AND after the catalytic and the catalytic is actually only 40k old, its a neuspeed full system, cat, downpipe and exhaust. Please reply ASAP.

 

Im assuming its a 1.8t? Cat is fine, and the o2 after the cat can be bad it doesn't effect how it runs, just throw a cel. But you should get a bank number that is telling you what 02 is bad. Have you clean the maf? unplugged the maf (so it can go to default settings.) Done another vag scan after it has started to run rough? Blinking cel?

 

I have not cleaned the MAF, the CEL went away though it never was blinking.

 

It is a 1.8T, it is struggling and rough idling so im going to change spark plugs tonight as well.

 

you will ONLY get a bank 1 fault because bank refers to the side of the motor affected.

4 cylinders only have 1 bank.

and its a good habit to change 02s when the cat is replaced. don't kno if it was... just saying. If they look old, then they prolly are. And lastly i have replaced countless aftermarket cats that don't last because they are garbage. Again i dunno, Just saying. Good Luck

 

Yes, they (cats) can be garbage but just don't go out over night. So the o2 sensor must be bank one then :P Sry, i though the bank was the position (before or after car). One thing I would try since the cel went away is just do a simple reset for the comp. Just unplug the battery for a few then plug it back in. But again, even if the second 02 sensor was bad, replacing wont make it run any different. Unplug the maf and see how it runs would be the second thing I would do.

 

I DID take off the MAF and the car runs sooo much better and it doesnt shoot the black smoke out, so does anyone else know what is causing me to be producing black smoke and it smells like unburnt fuel and lots of carbon deposits come out when connected. so in conclusion runs great withOUT MAF and horrible with it.
We change plugs and 02 sensor #1 and it covered them in carbon as soon as it was on.

 

i'd be thinking vacuum leak personally...

1. the 1.8t is notorious for them
2. only vacumm leaks throw CELs that disappear.


yea, the fault will say sensor 1 or sensor 2... BUT.... P0420 is a cat efficiency fault. which mean the numbers from sensor 1 to sensor 2 are too far off. it could be a bad sensor or just a misfire that would cause the cat eff to drop.

diagnosing from a computer is tough when you can't see, hear, feel, or smell anything