Hey guys. The girlfriend's car has been running great since the MAF was replaced, but now she came home and says the car is hesitating and the CEL was blinking.

Scanned it and got the following:

p0420 - Cat below threshold
p0171 - running lean
p0136 - 02 sensor circuit malfunction bank 1 sensor 2
p0300
p0301
p0303

So... I'm about to order a new 02 sensor, but since its the sensor downstream of the cat and the cat is throwing a CEL too, could it be the cat and not the sensor? Anyone had this problem before? Also, if I get a new o2 sensor, anyone have recommendations? Sorry not buying one from the stealership. Any Audi vendors that this forum supports?

 

ecstuning has a lot of decent prices on replacement parts

depending on what year/engine i think there might be two O2 sensors - not sure if they're the same or not but you'll want to make sure you get and replace the right one!

 

hate to break it to you but those are the exact codes i got when my maf went bad. did you replace the old one with a used unit, or maybe an aftermarket one?

 

Damnnnnnnnn. Had the MAF replaced at the dealer. God this car is by FAR the most problematic car I've ever delt with. Guess I'll start with the rear O2 and work from there. Thanks for the input guys.

EDIT: Anyone else feel a connection between the Cat cel and the rear O2 cel? Sucks that the MAF gives off a Running lean cel and I've got that too. =(

 

Start by looking for vacuum leaks. Spark plugs and coils should be checked next.

 

I'll check it over again for vacuum leaks, and I just put new plugs in. Is there any way to check a coil pack to see if its working correctly?

 

the rear O2 sensor has nothing to do with how the engine runs. it is to test the efficiency of the cat only. it is possible that a clogged cat is causing your problems, but replacing the sensor isnt going to do anything.

 

Thanks for all the help so far guys. I appreciate it as I have no experience with this car or Audi's in general.

ghost - Thanks I realized that after I posted. Most cars are the same but I also read it while searching around the forums for answers to this problem.

Anyone know of any conditions that have caused CEL p0136 to turn on when the cat wasn't actually bad? Probably going to look into new coil packs before I replace the cat because I know some of them are aged... And I'm checking out the MAF before I do anything.

 

P0301 and P0303 are misfires for cylinders # 1 and 3. Fix those first. Coil packs, spark plugs... Once you've figured those out, reset the codes and drive the car. The other codes could simply be a result of your misfires.

 

Tweaked - Thanks man. You definitely made me think straight.

I was under the impression from my g/f her car was barely running, so I never really started it up. I cleared the codes and swapped #2 and 3 coil packs (knowing the plugs have <1k) and started the car again. Obvious misfire, but when I scanned it, only cylinder #1 showed up (originally 1 & 3.) New coil pack for cyl #1 and she's running well again no engine light. I will be on the lookout for coil packs 2 and 4 to go bad soon. (The one i swapped from 2 to 3 i had replaced recently.)

Thanks again everyone. This is the second time this forum has proved to be a very helpful community. I appreciate everything and hope maybe I'll be able to give back one day. ;-)