Hi guys, ive been out of the audi/vw scene for a minute but i coulnt stay away forever. Even though these cars can really be a pain the **** sometimes, i always end up buying another one.

With that said. i bought a 2000 s4 2nd owner with all the receipts since the day it was new. (even the window sticker) both owners where women and th car is seriously mint inside and out.

car has 140k and after i bought it everything was fine. A egt sensor code triggered the cel, but i cleared it and never came back. one of the fans (inside one) went bad, and the outside one had the ground wire cut off for some weird reason so the car started to overheat a little while parked. Im guessing because of the my auxillary water pump blew. I had no time to change it so i took it to a meachanic to have it changed. Ever since than ive been getting the p0421 code.

i checked for leaks since they hvad to disconnect a bunch of stuff and i found a broken check valve which i fixed,and the line that goes brake booster leaking. Both problems have been corrected. Im just thinking if there could be something else that i havent found that they could have messed up and be causing the p0421 code?

 

Isnt 'P0421 - Bank 1 catalyst warm-up efficiency below threshhold'?

https://www.audiforums.com/forum/audi-a6-9/cel-code-p0421-70170/


I have found that sometimes this is set from using crap gas... I had it on a different car for months, but after switching stations, it went away.

 

yes it is, but i dont use anything but 93 octane gas. on my porsche 944 turbo i know the wastegate is actuated by the vac. line, so im guessing if the s4 is set up the same way than maybe the vac leak is affecting the function of the wastegate which might be affecting the warm up of the cat. It kinda makes sense to me????

 

Well, you could just swap out all the vac lines, but I would figure you'd get a different code to go with it if it was large enough to effect the cats.

 

As Mid said, I had that code a lot and switched gas stations and it never popped up again. Just food for thought; worth a try next time you fill up.

 

you know what im gonna give that a shot. The gas station ive been using is a local station. They have a few locations but not nearly as big as, hess, sonoco, mobil, or any of those.

 

has nothing to do your catalyst , youll get a drivability issue or a code for running rich or lean. p0421 is specifically dealing with the catalyst, possibly post cat 02 sensor.

 

im gonna order a 4 wire universal 02 sensor off of ebay, anyone know if theres a difference between bosch and bwd (borg Warner)? bwd are a little cheaper cost. If it works just as well i'd rather get that one.

 

as long as you wire them up properly they will work perfectly...

 

i dove an hour to school today, light came on like always. It comes on after 20-30 minutes of highway driving. On the way back i needed gas so i stopped at a shell station and filled up with shell V-power. 1 Hour drive home on the highway and no light???? Maybe this is the damn reason for the cel. Stupid gas stations with their watered-down gas. Ima give it a few more days before i call it a triumph or before i order an 02 sensor. ill report back if it comes back on.