This test ONLY applies to GM vehicles with the 12 pin ALDL OBD connector.
To check your computer for stored codes, open up a paper clip and insert one end into the A slot and the other end into the B slot. Be cautious of the orientation of the connector. In some models, its upside down and you would end up using pins G and H.
Now, sit in the drivers seat with a pad and pencil and turn the key to ON or RUN. Do NOT start the car.
The first code should always be 12. This is to tell you that the OBD system is operating correctly. If there are no other codes present, code 12 will repeat itself.
The check engine light will flash for each digit in a code. Take code 12. It will flash ONCE for 1, pause, and TWICE for 2.
FLASH, pause, FLASH FLASH. = 12
If there are 3 codes like 12, 14 and 21 will look like this...
FLASH, pause, FLASH FLASH = 12
Flash, pause, FLASH FLASH FLASH FLASH = 14
FLASH FLASH, pause, FLASH = 21
When its done with the last code, it will go back to code 12 and repeat.
To clear the codes. Turn the key to OFF, remove the paperclip, and disconnect both battery cables. Wait about 10 minutes and reconnect.