2002 A6 3.0. 160,000. I bought the car last month and took it to get an E-Check (Ohio). It failed with codes P0421 and P0431, Warm Up Catalyst Efficiency Below Thre.
The CEL is not on. When I turn the key to position II should I see the yellow engine CEL symbol light up?
The car runs fine, but the resonator has a small leak and car has an bit of an exhaust "growl" at low speed acceleration. I know the car may need a software flash, but before that. A few points, I am not driving it so not sure what gas has been in it. Will use 93 quality gas, check spark plugs and air filter.
1. How do I make sure the CEL is working properly?
2. Could the leaky resonator be causing these codes?

 

1.) As you mentioned before the CEL should light up right before the car starts. If the CEL never turns on in any of the key positions - it's probably a dead light.
2.) I don't think a resonator will cause both cat readings to fail - considering the resonator is behind the cat and rear o2s.

 

If these are the original CATs then you are already on barrowed time as the 3.0L CATs dont last long and usually go out in pairs. The reason your check engine light sisnt illuminate is because the fault is triggered during the warm up emission phase when the car is in open loop and once warm the car goes to closed loop and begins looking at the CAT efficiency and because they have now warmed up due to engine being hot they no longer fail the efficiency threshold test.

 

Sin, thanks for the reply. Update. It turns out the wires to both post cat O2 sensors were cut, and the CEL had been disabled. Real class act, but buyer beware I bought the car in a rush, I knew it needed some work and was priced accordingly. A shop reconnected the O2 sensor wires (a friend is using the car) and I guess they are working.
I should probably replace the O2 sensors first. Even if I have to get new cats, I would think those sensors are not going to work right?

 

I wish I could disable my CEL. MI does not check emissions and I have a piece of black tape on the "glass". The cats in my A6 failed around 185k and I replaced them with Billy Boats. Still a CEL. I replaced all the O2 sensors and it still came on. So I just live with it and wish I had saved the $2500 the cat work had cost me.


By comparison, the dealer replaced the cats on my daughter's 2003 A4 3.0 MT6 at 79k, just before the warranty expired. They are working without issue at 122k.

 

@ppgoal
I went to a local APR dealer and had my car flashed to stage 1. I asked my APR dealer to flash with the test-pipe file but there wasn't one for my engine type/model. So I placed a quick call to APR, they checked the database and said they can easily make that file available for my engine. 1 day later the file was uploaded and my local APR guy flashed the car with it. Haven't had a CEL for the rear o2's for a year and thousands of miles (completely removed the cats).
You should give APR a call and find out if they will do that for your car too.

 

I like your thinking, but it would probably be a little unreasonable to expect APR to flash code for my GIAC chip. :-) At this point, with 231k on the odo, I just put a piece of black tape over the CEL. That way it only annoys the passenger.