HIS4

My problems originally started with bad primary O2 sensors according to the VAG DTCs. So I changed both of them with Zoldman O2 sensors. About a week after I changed the O2 sensors I got this DTC:

16805 - Warm Up Catalyst; Bank 1: Efficiency Below Threshold

P0421 - 35-10 - - - Intermittent

I asked around the forum and some thought it could be the cats going bad. I haven't noticed a smell yet. I decided to just clear the code and see if it would come back. Well, everything was good until this weekend. About 3 weeks after clearing the code it came back. So now I need to figure out what this really is. Is there any values I could log that would give me a better idea of what's going on here?

S4KTS

Probably the rear O2's.

HIS4

Wouldn't the secondary O2s throw their own code?

berTTos@pjK04

grandma to reach temp. Apparently Audi made the ECU calculation between the 2 o2s too stringent and they did release updated code for this problem. STFA for it.

You'll see this DTC if you let your car idle to warm up or if you do a cold start to move your car then let it sit for a while then start it again and drive it. Precats don't warm up within threshold.

S40RCE:Smurf

it came up after couple weeks. Then stopped throwing a CEL and then it popped up again. Like berTTos said it is a idle thingy. Nothing major..

I think Audi has a emissions warranty 8 years or 80K miles that covers the CAT's or... you can just shave them and go with Test Pipes. I might look into that when the engine comes out for turbos soon.

I cleared it and it hasn't showed up yet..

S4KTS

They do have specific codes, but they can also cause the efficiency codes for pre-cats.

Swank4

2nd.