help locating/identifying part( leak detection pump??)

Does Any body know what or where this part is located on a 96 jetta(2.0 4cyl)? I took my car to the shop the other day because the check engine light was on. They said it was some leak detection pump or some thing like that and it is an expensive part. Could someone shed some light on this subject for me and would it be hard for a do it yourselfer to do?

Thanks for any info