On Vehicles before 1996 you could flash the codes on the dash. These vehicles operate with an OBD I System. On vehicles that are 1996 to 2005 You must use an automotive scan tool to pull the codes. These vehicles operate with OBD II System. And 2005 to present use CAN which also requires the use of an automotive Scan Tool.
For pre-1996 vehicles you can follow this link and select vehicles make and OBD I to find out how to flash the codes : How to Flash OBD I Codes
Just connect the scan tool into the diagnostic connector located under the dash and follow the instructions that came with the Scan Tool