Diagnostic Circuit Check

Fig. 014 - Wiring Diagram for Diagnostic Circuit Check.:

DIAGNOSTIC CIRCUIT CHECK

5.0L & 5.7L (CARB)

Circuit Description:
The purpose of the diagnostic circuit check is to; 1) make sure the "Service Engine Soon" light works, 2) the ECM is operating and can recognize a fault, and 3) to determine if any codes are stored. This is the starting point for any diagnosis.
When the engine is started, the ECM grounds terminal "G" to turn out the "SES" light. It alternately grounds and opens it to flash a code.

Fig. 013 - Chart - Diagnostic Circuit Check.:

Test Description:
Numbers below refer to circled numbers on the diagnostic chart.
1. Checks for the proper operation of the "Service Engine Soon" light. With the key "ON" and the engine not running, the light should be "ON" steady.
2. Grounding the test terminal will flash a Code 12 and any stored trouble codes. The light must go "ON" and "OFF", for a proper code. If the light goes from "Bright" to "Dim", this is not considered a code. See CHART A-6 in that case.
3. "NO CODES" at this point indicates that the fault is intermittent and the "System Performance Check" should be performed.