This is actually an a6 2.7t but similar to s4 and get more response here
Not sure if turbo('s) have gone or i got boost leak, have not tested for leaks yet as just so busy. boost used to hold 1.3 with lots of power, but now struggles to hit 0.8 even when it does in 3rd gear or above it dips down to 0.4 and wont come back up until i reset ecu or just restart engine only code i get is 17964 negative deviation or boost limit not reached..... iv attached 2 videos 1 is just mainly so you can hear the noise the turbos are making the other is the same but 3rd gear run with boost cutting down to 0.4.....
Boost 1 - YouTube
Boost test - YouTube
Boost 2 - YouTube