I have a 2000 Jetta TDI. I'm dealing with a P1557 code indicating Boost Deviation Positive. According to this site, it means my N75 may be bad/malfunctioning. Along with changing vacuum hoses, I'm planning to replace the N75 however it looks like I have two??? Which one do I replace? The one on the left has a black top and has part number "1J0 906 627" and the one on the right has a white top with part number "1J0 906 627A". Please advise. What do each of these do?