[Pneub]

I've got a P3Cars Vent gauge, and while I'm mostly using it to keep track of my oil temps, I've noticed the boost seems to be reading incorrectly. At idle I'm between -18 and -21, which seems about right, but at full boost it's only reading at 4psi when I'm actually boosting about 9. Since it's going through my OBDII I'm not sure why it would be reading so low. What am I missing (probably a lot)?