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Home >> GMC >> 2006 >> Sierra 1500 HD, RWD >> Repair and Diagnosis >> External Pages >> Different car >> Section 988 (HVAC System - Manual) >> Diagnostic Information and Procedures >> DTC P0646 or P0647 >> Circuit/System Testing

Circuit/System Testing

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  1. Ignition OFF, disconnect the A/C relay.
  2. Ignition ON, verify a test lamp illuminates between the relay coil B+ circuit terminal and ground.
    • If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.
  3. Connect a test lamp between the relay coil B+ circuit terminal and the A/C compressor clutch relay control circuit terminal.
  4. Engine running, command the A/C relay ON and OFF. The test lamp should turn ON and OFF when changing between the commanded states.
    • If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the ECM/PCM.
    • If the test lamp is always OFF, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM/PCM.
  5. If all circuits test normal, test or replace the A/C relay.
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