Carnesd

I just installed my autometer air/fuel guage and I comes on but even when I crank the car up I only get the far left green light on. I know that I hooked it up correctly, (only three wires). I think that I might have a bad o2 sensor because I hooked up a voltmeter to it and didn't get any reading. I also left it unplugged and the car runs the exact same as it did before. Would you guys say that this is the problem?