If you get a P1548 DTC code on your Audi TT, you may want to look at troubleshooting your N75 valve or even replacing it outright. In my experience, the cost of a replacement N75 valve is not much so it may be worth it to just replace it outright. I say this because even testing the N75 valve may show that it is working, but it may still be cause your P1548 code and be the contributer to your boost issues.