Code
C0387
HUMMER
C — Chassis
Unable to Perform Shift
Views:
UK: 7
EN: 11
RU: 7
AI status
Completed
Completed
100%
Causes
- Open or short in shift actuator wiring
- Failed transfer case/shift motor or actuator
- Faulty shift position sensor or feedback circuit
- Insufficient transfer case fluid or mechanical binding inside transfer case
- Poor battery voltage or ground at control module/actuator
- Faulty transfer case control module (TCM) or related ABS/BCM module
Symptoms
- Cannot change between 2WD/4WD modes or between low/high range
- “Service 4WD” or related dash message/warning lamp illuminated
- Shift command shown on scan tool but no actuator movement
- Grinding, clunking, or no sound when attempting to shift
- Vehicle stuck in a single drive mode
What to check
- Retrieve freeze frame and full scan tool data; note all stored DTCs and when they occurred
- Inspect fuses and relays for transfer case/4WD circuits
- Visually inspect wiring harness, connectors, and grounds at transfer case actuator and control module for damage or corrosion
- Check battery voltage and engine ground integrity with engine off and cranking
- Verify transfer case fluid level and condition per service procedure
- Attempt a shift command with a scan tool while observing actuator movement and live data
Signal parameters
- Shift request command (CAN) — present/absent
- Actuator supply voltage — key on: ~12V (12–14V)
- Actuator ground continuity — near 0 ohms to chassis when commanded
- Actuator current draw — expected low steady current; high spike indicates stall (compare to spec)
- Shift position sensor/feedback voltage — typically 0–5 V; should change when shifting
- Module supply and reference voltages — within manufacturer spec
Diagnostic algorithm
- Connect a capable scan tool; read and record all DTCs, freeze frame, and live data related to transfer case and CAN network.
- Check battery voltage (engine off and cranking) and verify good grounds at chassis and control modules.
- Inspect fuses, relays, connectors and wiring to the transfer case actuator and position sensor; repair any damage or corrosion.
- With scan tool, command a shift and observe: a) whether the module sends the request, b) actuator supply voltage and current, c) feedback/position sensor change.
- If actuator receives voltage but does not move, measure actuator resistance and apply battery voltage (bench or controlled) to verify motor/actuator operation. Replace if inoperative or draws excessive current.
- If actuator works but module reports no feedback, test/replace the shift position sensor or check its circuit for opens/shorts.
- If there is no command from the control module, verify CAN bus communication and related modules (ABS, BCM) for U-series or communication DTCs; repair CAN wiring or faulty modules as needed.
- Check transfer case fluid level and inspect for internal mechanical blockage; service or repair mechanical fault if found.
- If electrical and mechanical components test good, check for available software updates or reprogram the transfer case/TCM per manufacturer procedure.
- Clear codes and road-test to confirm repair; re-scan for reappearance of C0387 or related codes.
Likely causes
- Damaged connector or corrosion at transfer case actuator
- Shift motor draws excessive current (stalled)
- Shift position sensor out of calibration or failed
- Intermittent power/ground to actuator or control module
- CAN bus message for shift request not being received by transfer case module
Fault status
Status
Control module reports inability to execute requested shift — the shift actuator did not achieve the commanded position (electrical, mechanical, or communication fault).
Repair difficulty: Medium
Diagnostic time: 1.0 - 3.0 hours
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