Diagnostic Circuit Check

Fig. 002 - Trouble Code Identification.:

DIAGNOSTIC CIRCUIT CHECK

4.3L VIN 'T' & 5.7L VIN 'N'

The purpose of the Diagnostic Circuit Check is to (1) make sure the "SERVICE ENGINE SOON" light works, (2) make sure the ECM is operating and can recognize a fault, and (3) to determine if any trouble codes are stored. The memory is cleared on the Diesel Electronic control system any time the ignition switch is turned off. Therefore, unless the engine is left running when the vehicle is brought in for a complaint or the code is reset during this diagnostic circuit check, the problem should be considered intermittent. This is the starting point for any diagnosis. If there are no codes indicated, go to the Driveability Symptoms.

Fig. 001 - Diagnostic Circuit Check.:

1. Check for proper operation of the "SERVICE ENGINE SOON" light with the key "ON", 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-1 in that case.
3. This step is to determine a fault is still present, or was intermittent and is no longer there. Memory is cleared when the ignition is cycled off and the engine is run for 30 seconds to see if trouble code(s) will reset.
4. If the light is "ON", fault is still present; therefore, go to the applicable trouble code chart.
5. If the light is "OFF", the fault is either intermittent, or it is a code that cannot be set in the stall (I.E. 24 or 41). For codes that cannot be set in the stall during the Diagnostic Circuit Check, the vehicle may be run on hoist or road tested. VIN N, VIN T