I would start with the vacuum tubing. My car is an S8 so I don't know specifically how the system on your car is set up but I have had 2 secondary air injection faults over the last 2 years and both were leaking vac pipes. In the end I replaced all the pipework. A vac pump would be useful to test the pipe work and VCDS to output test the pump if you have access to them.
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Audi S8 D2 2002 88k mls
Ebony pearl effect black. Grey interior.
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