In order to scan your car for codes, you must first locate the diagnostic connector.  
It will be under the drivers side dash. Here is an example of its location. Your  
vehicle may be in a little different spot.
If you do not have a car code reader, many of the large national parts store chains  
will scan your cars computer for free.
On todays newer vehicles (1996-and up, OBDII), the diagnostic terminal is what  
powers up the scan tool. If the scan tool does not power up after plugging in to the  
connector, you will need to check fuses. Different vehicles use different fuses to  
power this connector.
After reading the codes(s) and the short description, you will need to use a diagnostic  
chart to pinpoint the problem. Just because the scanner shows, lets say, a code for the  
intake air temp.sensor, does not necessarily mean that the sensor is bad.
The most basic scanners will just read trouble codes from the Powertrain Control  
Module (PCM). A little higher priced scanners will read codes, and give some live data  
streams. The high end scanners will do this, and read other computers such as Body  
Control Module(BCM), Anti-lock Brake module(ABS), Airbag module(SIR), Door  
modules, 4-wheel drive modules, etc. These will also be able to control devices in a  
given circuit to go along with diagnostic charts, and run pre-programmed test, and  
re-program replacement computers.
Schematics are used after getting the trouble codes to help in diagnosis.
Location of the ALDL connector to check for GM computer trouble codes.
Scan tool installed to read computer codes.
Need help with your code?
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How to scan for computer (PCM)
DTC codes on GM vehicles
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