I have Audi A6 1999, 2.8L Quattrro. I had a check engine light ON and read the codes thru VAG. It came up with a long list.

2 Faults Found:
16806 - Main Catalyst: Bank 1: Efficiency Below Threshold
P0422 - 35-00 - -
16795 - Secondary Air Injection System: Incorrect Flow Detected
P0411 - 35-10 - - - Intermittent
Readiness: 0000 0000

1 Fault Found:
00526 - Brake Light Switch-F
27-00 - Implausible Signal

6 Faults Found:
01572 - Alarm triggered by Door Contact Switch: Passenger Side
35-00 - -
01370 - Alarm triggered by Interior Monitoring
35-00 - -
01369 - Alarm triggered by Hood Switch
35-00 - -
01368 - Alarm triggered by Luggage Compartment Switch
35-00 - -
00955 - Key 1
09-10 - Adaptation Limit Surpassed - Intermittent
00956 - Key 2
09-10 - Adaptation Limit Surpassed - Intermittent

I know the bottom 6 may be due to some error from my side as I was checking the VAG tool and it did trigger some alarm once in the car. But I am really worried about the top 2.

When I start the car, I get a clinking sound from right below the drivers seat. Sounds like something is loose inside the CAT. Can anyone help me identify the exact issue.

If its the CAT, how much would it cost and where do I get it done. No Dealers please. Is it DIY?

 

not a problem just clear the codes and drive a few cycles to reset the ECM then check for codes, chances are the test was done premature to the required standards

 

I cleared all the codes and will see if any comes back. BTW what do you mean by one or two cycles. how log are they typically. If its in miles, I drive about 30 miles a day on the car.