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Home >> Chevrolet >> 1991 >> Forward Control 7.4 N >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Tests W/Codes - Transmission >> Self-Diagnostic System >> Diagnostic Procedure >> Diagnostic Procedure
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Diagnostic Procedure

  1. Ensure all non-controlled systems not related to engine or transmission control system are operating properly. DO NOT  proceed with testing unless all other non-controlled system problems are repaired.
  2. ALWAYS begin system diagnosis with checking diagnostic circuit. See DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article in this section. This determines if the computer-controlled system is working properly. If trouble codes are displayed (other than Code 12), determine if codes are hard or intermittent trouble codes.
  3. A hard code is one which is present while working on vehicle, and problem still exists. 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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