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Home >> GMC >> 1991 >> Forward Control 6.2 J, Automatic >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Tests W/Codes - Diesel >> Self-Diagnostic System >> Diagnostic Procedure >> Diagnostic Procedure
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Diagnostic Procedure

  1. Ensure all engine systems NOT related to the Diesel Electronic Control (DEC) system are operating properly. DO NOT proceed with testing unless all non-DEC system problems are repaired.
  2. ALWAYS begin diagnosis with checking diagnostic circuit to determine if the DEC system and ECM are working properly. See DIAGNOSTIC CIRCUIT CHECK chart. If trouble codes are displayed (other than Code 12), determine if codes are hard or intermittent trouble codes.
  3. A hard code is one that is present while working on vehicle, and the problem condition persists. Hard codes will cause the SERVICE ENGINE SOON light to come on.
  4. An intermittent code is one which does not reset itself and is NOT present while working on vehicle. Intermittent codes are often caused by loose connections. The SERVICE ENGINE SOON light will go out 10 seconds after fault goes away. For intermittent diagnosis procedures, see TESTS W/O CODES article in this section.
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