Don't waste your money thinking a "plug in" will tell you anything it won't, this is basic fault finding, just get a haynes manual for high level sierra, ghia etc and follow along route.
The first place to look is at the connector that joins the ecu to the car loom and from where it gets it's power/sends power out to fuel pump etc.
At the rear of the dash above glovebox just under the screen area(inside car) is a connector quite well hidden, with 6 wires in it.
thin green = tacho, red/blue = permanent live, black = switched live(from ignition switch), black/red = fuel pump live output(putting a live directlty to this will work the pump), brown/white = temp gauge, brown/red = air con(if fitted)
tabetha