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Dan, Auto Technician
Category: Chevy
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Experience:  GM Mark of Excellence/ Service Guild Participant with 13 years experience, ASE Master Certified
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2002 trailblazer: lights working..No service 4wd warning light..2wd

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2002 trailblazer - 4wd switch - no lights working on switch. No service 4wd warning light. Won''t switch from 2wd. Unhooked battery, waited 5 minutes, hooked back up. 4wd service warning light came on.
Subject:Inoperative 4WD/AWD Lamps, Inoperative 4WD/AWD System, DTC C0550 Set No Communication with TCCM (Reprogram Transfer Case Control Module) #02-04-21-006E - (07/20/2006)


Models:2004 Buick Rainier
2002-2004 Chevrolet TrailBlazer, TrailBlazer EXT
2003-2004 Chevrolet Avalanche, Silverado, Suburban, Tahoe
2002-2004 GMC Envoy, Envoy XL
2003-2004 GMC Sierra, Yukon, Yukon XL
2004 GMC Envoy XUV
2002-2004 Oldsmobile Bravada
with Electronic Shift or Active Transfer Case (RPOs NP1, NP4 or NP8)


This bulletin is being revised to add Subject information and change the labor operation. This bulletin cancels and supersedes Bulletin Number 03-04-21-006. Please discard Corporate Bulletin Numbers 02-04-21-006D and 03-04-21-006 (Section 04 -- Driveline/Axle).


Some customers may comment that the 4WD/AWD indicator lights on the selector switch do not work, or the 4WD/AWD system may be inoperative and/or the service 4WD light is illuminated. The condition is typically intermittent and always occurs at key-up. Upon investigation, the technician may find DTC C0550 set.


The Transfer Case Control Module (TCCM) has not received the wake-up call during key-up and has remained in the "sleep mode".

When the TCCM is in the "sleep mode", it will not communicate with the class II bus or the Tech 2®.


  1. Try to communicate with the TCCM. If no communication, go to step 2. If communication is possible, go to step 5.
  2. Remove the underhood (ATCM or TREC) TCCM fuse.
  3. Wait 30 seconds.
  4. Reinstall the fuse. The module should "wake up" and communicate with the Tech 2®.
  5. Check for codes in the TCCM. If a C0550 is found current or in history with or without other codes, replace the module and go to step 6.
  6. Reprogram the TCCM with the latest TIS software using normal SPS procedures.
  7. Verify operation and that no codes are present.

Important: For the 2002 model year, the TCCM must be replaced and reprogrammed.

Warranty Information

For vehicles repaired under warranty, use:

Labor Operation


Labor Time


Transfer Case Module - Reprogram with SPS

0.4 hr


GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information.
Subject:4WD Switch Indicator Lights Flashing, 4WD Inoperative, DTCs C0327, C0550 Set (Replace Transfer Case Encoder Sensor, Reprogram TCCM) #04-04-21-004 - (12/16/2004)


Models:2004-2005 Buick Rainier
2002-2005 Chevrolet TrailBlazer, TrailBlazer EXT
2002-2005 GMC Envoy, Envoy XL
2004-2005 GMC Envoy XUV
2002-2004 Oldsmobile Bravada
with Four-Wheel Drive (4WD)



Some customers may comment that the Service 4WD light is on, the 4WD system is inoperative or the 4WD switch indicator lights are blinking.

Service Diagnosis

Technicians may find DTC C0327 in the Transfer Case Control Module (TCCM). This would indicate an encoder sensor malfunction.

It is also possible that the module may be in a mode that will not allow any shifts or updates for the software or calibrations. If this occurs, the TREC fuse (in the engine compartment) must be pulled and reinstalled after 30 seconds. This should allow further operation and service of the system. If a C0550 code is found, this is an unrelated concern and SI diagnostics should be followed for this concern.


Important: Carefully handle and install the new encoder. Improper installation of the encoder could result in a repeat failure.

  1. Raise the vehicle.
  2. Disconnect the transfer case motor/encoder wiring harness.
  3. Remove the four bolts that secure the motor/encoder to the transfer case.
  4. Remove the motor/encoder from the transfer case.
    Object Number:(NNN) NNN-NNNN Size: SH
    Click here for detailed picture of above imageidth=
  5. Remove the black plastic gasket/over plate from the motor/encoder.
    Object Number:(NNN) NNN-NNNN Size: SH
    Click here for detailed picture of above imageidth=
    5.1. Push out the two "single" locking tabs (two lower arrows in illustration above) located on the underside of the housing (motor end).
    5.2. At the other end of the gasket/cover plate, place a flathead screwdriver tip (1) between the gasket/cover plate and the motor housing near the "dual" locking tabs. Twist the screwdriver clockwise/counterclockwise at both sides until the locking tabs break and the gasket/cover plate comes off.
    5.3. Discard the gasket/cover plate.

    Object Number:(NNN) NNN-NNNN Size: SH
    Click here for detailed picture of above imageidth=
  6. Avoiding the wire connection, take two small flathead screwdrivers (one on each side of the encoder) and rock the encoder sensor out using light pressure on each screwdriver.
  7. Gently remove the encoder sensor from the wire harness connector. Pry these apart by inserting a small flathead screwdriver between the terminals and the harness connector. Verify that no damage or corrosion exists on either male encoder terminals or female wiring harness terminals.
    Object Number:(NNN) NNN-NNNN Size: SH
    Click here for detailed picture of above imageidth=
  8. Install the new encoder sensor by connecting to the harness and then gently sliding the encoder sensor over the motor shaft. The hex feature (3) on the encoder should face down toward the motor housing. Make sure the tab (1) on the encoder mates with the notch (2) on the shaft. If the encoder tab is not aligned with the output shaft, rotate the encoder inner ring by hand to mate properly. Press the encoder fully onto the shaft and into the housing.
    Object Number:(NNN) NNN-NNNN Size: SH
    Click here for detailed picture of above imageidth=


    Make sure the encoder cover (1) is flush or below the surface of the motor housing and fully seated into place.
    Ensure that the new encoder/harness assembly is fully seated to the encoder sensor.


    Object Number:(NNN) NNN-NNNN Size: SH
    Click here for detailed picture of above imageidth=
  9. Install a new gasket/cover plate. This should snap into position. Make sure all the four tabs are FULLY engaged.
  10. Install the motor/encoder onto the transfer case using four M8-1.25 X .35 flange head bolts.

    Tighten all four bolts to 13.5 to 17.6 N·m (10 to 13 lb ft).

  11. Connect the transfer case motor/encoder wiring harness.
  12. Following normal SPS reprogramming procedures, reprogram the transfer case control module with the latest software calibration.
  13. Verify that the 4WD system is operational.

Parts Information

Part Number



Actuator Asm (NP4)


Actuator Asm (NP8)


Gasket/Cover Plate


126/226 AB Encoder Sensor

Parts are currently available from GMSPO.

Warranty Information

For vehicles repaired under warranty, use:

Labor Operation


Labor Time


Sensor Assembly, Transfer Case Range Selector - Replace

0.7 hr*

*This labor time includes reprogramming.


with skid plate

0.2 hr


Diagnosis time

0.0-0.7 hr


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