If your vehicle is a model year 1995 or older, you will be able to check OBD-1 diagnostic trouble codes without using a dedicated scan tool. 1996 and newer vehicles operate with OBD-2 and require the use of a scan tool.
Verify the Initialization Procedure
To access OBD-1 codes, the vehicle's computer must be set to a special mode that allows display of the stored engine codes. The process varies by vehicle make, and can be found in the car's service manual or online. The procedure may involve connecting a disconnected electrical connector, following a sequence of pumping the brakes, gas pedal or some other action before turning the ignition to the accessory position.
Read the Diagnostic Trouble Code
When the vehicle's computer is in diagnostic mode, the malfunction indicator light (MIL, or check-engine light) will flash a series of short and long pulses separated by pauses. Digits of the code are deciphered by counting the flashes of the light. The first digit will be composed of long pulses, the next digit with short pulses, and so on until a pause is encountered. Pauses signify the end of a code. The meaning of these codes varies by vehicle make.