Oil Pressure Sensor 456180 Readings

I am configuring a new 914 engine with a Garmin G3X.  When using the oil pressure sensor (#456180) in a Garmin G3X touch setup, my oil pressure reading in an engine off condition shows 15 psi.  Even took the sensor out of the engine and still had 15 psi.  First thought a faulty sensor, so I tried another one and it does the same thing.

I have been communicating with the Garmin G3X experts on this and have reviewed everything they came up with.  Wiring is good.  Resistor in place on the GEA24.  They are running out of ideas.  This got me to thinking and doing some additional research.

1. Ambient air pressure at sea level is about 14.69 psi.  Field elevation is about 900 so it could be conceivable that the sensor is reading ambient pressure.

2. Took sensor out of engine and used a syringe to create a slight vacuum on the sensor.  The reading then showed "---" meaning the pressure was below the approximately 11 psi minimum that the sensor can read.  Removed the syringe, and the pressure returned to 15 psi.  Added pressure and the reading went above 15 psi.  Removed and it went back to 15.

Is my theory correct for ambient air and is this how your G3X system reacts (current software version of 8.30)?  Just trying to wrap my head around on how to identify a no pressure situation.  At altitude, it probably become evident as the pressure will be less than 11 I'm guessing but what about ground operation.  I don't want a false reading of 15 when I have no pressure when the minimum below 3500 rpm is 12.  Or does ambient air pressure become irrelevant once the engine is running.