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Home >> Chevrolet >> 2020 >> Express 2500 LS, 6.0L Eng VIN G >> Repair and Diagnosis >> External Pages >> Different car >> Section 79 (HVAC - Manual) >> Diagnostic Information and Procedures >> DTC P0645, P0646, or P0647 >> Circuit/System Testing

Circuit/System Testing

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  1. Ignition OFF, disconnect the A/C compressor clutch relay.
  2. Verify that a test lamp illuminates between the ignition circuit terminal 86 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 the ignition circuit fuse/breaker is open, test or replace the A/C compressor clutch relay.
  3. Connect a test lamp between the ignition circuit terminal 86 and the control circuit terminal 85.
  4. 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 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 compressor clutch relay.
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