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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 other non-DEC system problems are repaired.
  2. The DEC system diagnosis should ALWAYS begin with the DIAGNOSTIC CIRCUIT CHECK chart. This determines if the DEC system and ECM are 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 when working on vehicle and the condition 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 H - TESTING W/O CODES article in ENGINE PERFORMANCE.
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