I just purchased 2003 Audi Avant A6 3.0, I got an error code of 421, which means low temperature catalytic flow. Could someone please help me understand this code. Any help would be appriciated. TIA,

http://www.obd-codes.com/p0421

How many miles are on your car?

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Jeff in Minneapolis
'00 A6 Avant Quat 145k
100% stock

105,200

If you DIY then clear the code and see if it comes back.
Next, change the o2 sensor & spark plugs
lastly, change the cat

The dealer did/does change the threshold (or something) for the cats, I have no idea if it applies to your issue, but you could call and ask.

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Jeff in Minneapolis
'00 A6 Avant Quat 145k
100% stock

Thanks Jeffla,
I had the code cleared out yesterday and it has not come up again. I can change the sparks plug easy so I'll do that next. The O2 sensor, have no idea about where to look for it.

if the codes didn't come back why change anything?

More commonly your code will come back every 20-500 miles at random points I found and is messing up my performance. I know myO2 sensors are on the last leg but need to check my sparkplugs :P

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2001 audi a6 2.7T(tiptronic) Stage3
Tuning GIAC/AWE custom tune, GIAC/AWE tipchip
Support APR bipipe, APR snub mount
-Forge diverters, Fuel pressure pump
Intake Full Dar intake
Exhaust piggy downpipes, 3.5' APR exhaust
-Genuine RS4 Engine Mounts
-awe-tuning intercoolers
Coming Soon...H&R Coilovers

How bad is it to change O2 sensors?

wammy18 you were right, I had the check engine light pop up again. Any suggestion on O2 sensor replacement?

bahhh I have been holding off aswell because I have court coming up where I might not be able to drive anyways But I will ask around and ask my buddy with an s4.. I know parts for the o2 sensor $215.00.... Sadly I have been pretty stupid with getting stage 3 and not checking my spark plugs!! Def do this when replacing or checking the parts. That is fairly easy to do but let me know if you need a step by step with checking the spark plugs. Takes about10-20 mins to check since I have taken the intake out who knows how many times

__________________
2001 audi a6 2.7T(tiptronic) Stage3
Tuning GIAC/AWE custom tune, GIAC/AWE tipchip
Support APR bipipe, APR snub mount
-Forge diverters, Fuel pressure pump
Intake Full Dar intake
Exhaust piggy downpipes, 3.5' APR exhaust
-Genuine RS4 Engine Mounts
-awe-tuning intercoolers
Coming Soon...H&R Coilovers