Had a problem with the oil pressure gauge in a Tiger. So I called Mitchell Gauges (Aircraft) in Illinois and talked to Bob in Technical Support. I knew the oil pump was working via a mechanical gauge hooked into the oil line. That meant it was either power, gauge or pressure transducer. According to BOB here is how to troubleshoot:

5 ohm resister will read 0 on all their pressure transducers regardless of range.
180 ohms will read high value, 90-100 approximately midrange.
Easiest way to test, take the lead OFF the transducer which will cause the gauge to peg high. This will not hurt the gauge, but will verify the gauge and the power to it.

FYI, it was the transducer, new one on order from Aircraft Spruce.