Any advice on where to start looking for the reason for this fault?

<center><img src="http://i255.photobucket.com/albums/hh156/ricoblanco/AIRsystemA6.jpg"></center><p>
1 - Engine Control Module (ECM)
2 - Air Cleaner (ACL)
3 - Secondary Air Injection (AIR) pump motor -V101-
4 - Connecting tube From air cleaner to secondary air injection pump
<b>5 - Vacuum line</b>
6 - Connecting tube, right From combination valve to exhaust manifold
7 - Connecting tube From secondary air injection pump to combination valves
8 - Combination valves
9 - Connecting tube left From combination valve to exhaust manifold
10 - Left exhaust manifold
11 - Right exhaust manifold
12 - Secondary Air Injection (AIR) solenoid valve -N112-
13 - Vacuum reservoir


You should also look at the connecting tubes as well.