Sorry 2002 a6 quattro 3.0 avant
i do have codes also ,
p0421
po431
p0442

as i said previously i just bought this car it ran great then got a check eng light,,couple days later it just started running real rough, missing , no power.
dont want to screw it up so i towed it home any ideas? or questions for me that might help you get a grip on whats going on i would really be thankfull

 

First your thread title says "a5 3.0..." your having a problem with an A6. Second the year model you have does make a difference. Third if you have code numbers give them to us so we can look them up if we have the ability to. These three things will help you get answers quicker. Dont mean to come off as rude just trying to help you get the answers you seek!

 

sorry my first post was a typo,
but i do have the codes

p0421
p0431
p0442

and its running rough ,sounds like its missing on a cylinder or somthing
and this started a couple days after getting the check eng light.

 

I have the service manual that should have the P codes for your motor in it but P0421 and P0431 dont appear on the list. P0442 is EVAP emmision control system leak. Not sure what the other two could be since they are not listed in the Bentley service manual.

 

The following are your codes:
P0421-Warm up Catalyst Bank 1: Efficiency Below Threshold.
P0431- Warm up Catalyst Bank 2: Efficiency below threshold
P0442-EVAP Emission Control System. Small Leak

Rosstech suggest a possible firmware update for the vehicle.
The other thing to consider is the catalytic converter could be faulty. I know you can test the sensor at the catalytic converter with the right scan tools- I'm not sure of the procedure with the Audi or this engine but I just performed this check on a Cadillac DTS a couple days ago, I was able to test the sensors before and after the catalytic converter and had base numbers to compare those too to help determine a faulty catalytic converter.

 

Also it could be possible, (I'm not a 100% sure on this vehicle) if just tour EVAP System is leaking and throwing the other two codes in relation to the EVAP code. I know the most common reason for this is for the gas cap not being closed properly. CHECK YOUR GAS CAP!