Without Scan Tool

If a suitable tester is not available, stored fault codes can be accessed directly through the check engine lamp on 1987---88 models. To call up codes, cycle ignition switch on, off, on, off and on within 5 seconds. Stored codes will be indicated by flashes of the check engine lamp.
Fault codes indicate the result of a failure, but do not identify the reason for failure. In most cases proper diagnostics cannot be determined by only using fault codes.
Once the check engine lamp begins to display fault codes, it cannot be stopped. If you lose count, it will be necessary to start over again. On 1988 models, the lamp cannot display fault code 16.
After all codes have been displayed on 1988 models, the switch function can be verified. The lamp will turn on or off as a switch is turned on or off.