box, sensor, or guage?

[skilletky]

just put a used FMS box ,that i got from a member on here, on my 01. works awesome, but noticed that my boost guage (autometer) will be reading about 6 psi, but the FMS box will read 0psi. there is a constant 5-6 psi differance between the 2. i dont know which one to go by, or if the guage, the box, or the MAP sensor has issues. i searched but couldnt find any other similar problems.