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Home >> GMC >> 2006 >> Sierra 1500 HD, RWD >> Repair and Diagnosis >> External Pages >> Different car >> Section 1138 (HVAC System - Automatic) >> Diagnostic Information and Procedures >> DTC P0645, P0646, or P0647 >> Circuit/System Testing

Circuit/System Testing

WARNING: This page is about a different car, the 2008 Cadillac CTS. However, it is still accessible from the selected car via links, so may be relevant.
  1. Ignition OFF, disconnect the A/C COMP relay.
  2. Ignition ON, verify that a test lamp does not illuminate between the relay coil control circuit terminal G7 and ground.
    • If the test lamp illuminates, test the control circuit for a short to voltage.
  3. Verify that a test lamp illuminates between the relay coil ignition circuit terminal E8 and ground.
    • If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance. If the circuit tests normal and ignition circuit fuse is open, test or replace the relay.
  4. Connect a test lamp between the relay coil ignition circuit terminal E8 and the relay coil control circuit terminal G7.
  5. Command the A/C relay ON and OFF with a scan tool. 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 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 PCM.
  6. If all circuits test normal, test or replace the A/C COMP relay.
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