I tought that when hooked up, the computer would run a test.
And tell him what sensor is not working properly.(Bad,low voltage)
What u guys think?
No, that isn't exactly how it works, the mechanic has to have the knowlege and ability to decipher what the car is displaying on the scan tool. It sounds like he just doesn't have that ability. Are you taking it to a tuner, or a dealer or an independant or what?

If it helps any, almost all engine data sensors recieve a 5-volt reference voltage from the PCM. A knock sensor only has one wire, if it is shorted to ground the PCM will think it is knocking and retard the timing. The crank and cam sensors are simple hall-effect switches, which actually creates a digital sign wave and send it to the PCM so it knows the position of the Crankshaft and Camshaft for Ignition timing, and injector sequence. If your engine starts and Idles, you can rule out a faulty Crank or Cam sensor in most cases.