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Diagnostic Procedure

WARNING: This page is about a different car, the 2003 GMC Sierra, 2003 GMC Cab & Chassis Sierra, 2003 Chevrolet Silverado, and 2003 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
  1. Did you perform the diagnostic system check? If so, go to next step. If not, see DIAGNOSTIC SYSTEM CHECK  under SELF-DIAGNOSTIC SYSTEM.
  2. Install the scan tool. Turn ignition on, engine off. Record the DTC failure records. Using the scan tool, monitor 4WD low status and select 4HI followed by 4LO with the transfer case selector. Does the scan tool indicate 4WD low status as NO when 4HI is selected, and YES when 4LO is selected? If so, see DIAGNOSTIC AIDS . If not, go to next step.
  3. Select 4HI followed by 4LO using the transfer case selector. Does the scan tool indicate 4WD low status as YES for both selector positions? If so, go to next step. If not, go to step  9.
  4. Remove wiring harness connector at the transfer case selector. Does the scan tool indicate 4WD low status as YES? If so, go to next step. If not, go to step  7.
  5. Inspect 4WD low signal circuit (1694) for a short to ground. Did you find a condition? If so, go to next step. If not, go to step  15.
  6. Repair the short to ground in 4WD low signal circuit. Is the repair complete? If so, go to step  16.
  7. Inspect the transfer case switch for proper operation or a short to ground. Did you find a condition? If so, go to next step. If not, go to next step. If not, go to step  15.
  8. Replace the transfer case switch. See TRANSFER CASE SWITCH & TRANSFER CASE SHIFT CONTROL MODULE  under REMOVAL & INSTALLATION. Did you complete the replacement? If so, go to step  16.
  9. Select 4HI followed by 4LO using the transfer case selector. Does the scan tool indicate 4WD low status as NO for both selector positions? If so, go to next step. If not, go to step  15.
  10. Disconnect wiring harness connector at the transfer case switch. Ground 4WD low signal circuit to a known ground. Does the scan tool indicate 4WD low status as NO? If so, go to next step. If not, go to step  13.
  11. Inspect for open in 4WD low signal circuit. Did you find a condition? If so, go to next step. If not, go to step  15.
  12. Repair the open condition in 4WD low signal circuit. Is the repair complete? If so, go to step  16.
  13. Inspect the transfer case switch for an open condition. Did you find an open condition? If so, go to next step. If not, go to step  15.
  14. Replace the transfer case switch. See TRANSFER CASE SWITCH & TRANSFER CASE SHIFT CONTROL MODULE  under REMOVAL & INSTALLATION. Did you complete the replacement? If so, go to step  16.
  15. Replace the TCM. See TRANSMISSION CONTROL MODULE  under REMOVAL & INSTALLATION. Is the replacement complete? If so, go to next step.
    CAUTION: In most cases, the TCM is not at fault. Investigate thoroughly before replacing the TCM.
  16. Perform the following procedure to verify the repair:
    • Clear the DTC.
    • Drive the vehicle under normal operating conditions.
    • Monitor the 4WD low status with the scan tool.
    • The 4WD low status must indicate NO when 4WD HI is selected, and YES when 4WD LO is selected.

    Did the DTC return? If so, go to step  1. If not, system is okay.

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