Checker and autozone type places will pull the codes for you for free most the time. Because they will sell you the part to fix it right there too.
If you want to do it yourself, no problem. Follow these steps : EXTRACT GM TROUBLE CODES
You will find step by step procedures for extracting the codes yourself along with code descriptions and causes for each code.
EXAMPLE:
CODE 24
Trouble Code 24 indicates that the Vehicle Speed Sensor (VSS) is reporting an abnormally low reading when other sensors indicate it should be higher.
The conditions for setting this code are:
1) no Code 33 or 34 (MAF Error) is present, and
2) the vehicle speed signal indicates < 4 MPH and
3) engine speed is between 1400 and 4400 RPM, and
4) during a low load condition (LV8 between 50 and 99), and
5) transmission not in Park or Neutral, and
6) all conditions met for more than 20 seconds
If the code was logged when the vehicle was in motion, the following should be checked:
1) Defective or corroded VSS connections
2) Maladjusted TPS sensor. The TPS should read about 0.42 volts at closed throttle
3) Defective speedometer cable
4) Bad or missing speedo drive gear(s) in tail shaft housing of the transmission
5) Defective VSS
6) Defective ECM
