Here's how I got my boost gauge to work:
Any square boost sensor part number: 25085-89900 from S14, Z32, R32 or R33 will work, if not, check connections or try a different one as my first one didn’t work.
Newer round boost sensors from R34 etc will NOT work with this boost gauge.
Here's wiring diagrams for the R33 GTR triple gauge cluster that I used to get it working. (same info as S14 for boost gauge):
Enjoy
Any questions just ask