Audi Repair: Secondary air injection system, solenoid valve, air injection

Question
I have a 2002 Audi A4 1.8T Quattro.  I am having problems with fault 16795 -- Secondary air injection system:Incorrect flow detected.

I have had the hoses checked and replaced, the secondary air injection solenoid valve replaced and the secondary air injection pump replaced and I still keep getting the same code on the car.

This has been going on for a month.  The shop thinks they fix it.  Hold on to it and test drive it for days.  I get it for 2 days and the problem comes back.

Any other ideas to look at?

Answer
hello
What I do at this point, I will make sure that the pump runs. You can or the shop can run the test and see if the pump comes ON.( I don't even know why they don't test the whole system before giving back the car.) If is OK, vacuum has to be checked at combi valve (at the back of the head) when the tast is ON vacuum has to be there. If the pump runs, vacuum gets to the valve but the system fails and set the fault, the COMBI valve is not opening (incorrect flow)
good luck                           pltech