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Service 4wd Message And Scan Codes


2003 silverado halfton- been getting the service 4wd message each time I start my truck, and I can't shift out of Auto4wd. Also, whlie in Auto4wd, when the back wheels spin, the front don't engage so all-in-all, the 4wd is not working. I took my truck to a gm dealership to scan for codes and it came back with 10, YES 10 different codes that were coming from the transfer case he said. I would like some advice as to where I should start. He said that most times if the code B2725 is thrown, in MOST cases it is the dash switch. But why am I getting so many others? Someone please help!


BO790-transfer case nuetral indicator circuit malfunction

CO550-ECU malfunction

CO379-front axle control circuit malfunction

CO374-general system malfunction

CO359-discrete output circuit malfunction

CO306-motor A/B circuit malfunction

CO300-rear propshaft speed sensor malfunction

U1301-class 2 communication circuit low volts

B2725-mode switch circuit malfunction

U1000-class 2 data link malfunction

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First.... check your fuses!! :cheers:


Second... I'd try this:


Bulletin No.: 02-04-21-006E


Date: July 20, 2006




Inoperative 4WD/AWD Lamps, Inoperative 4WD/AWD System, DTC C0550 Set, No Communication with TCCM (Reprogram Transfer Case Control Module)



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.



Third.... I'd clear all of the codes and see what comes back. Some of those could have been from previous work done on the truck and are not "active" codes.

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thanks sk1er18. I just tried removing that fuse, had it out for 30 seconds and put it back in then started the truck and it didn't do anything. Also, I checked the fuses, they all checked out OK but I replaced them anyway. I could have the codes reset, but I don't know if I want to spend another $60 for diagnostics again. What would you say that I start with? A new switch is only $48 from gmpartsdirect.com. If that doesn't work, do I move to the front diff actuator, or the transfer case encoder motor on the side of the TC? Thanks

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With all of those codes you'll end up spending $1000+ throwing parts at it. They should have cleared the codes to start with.....


Only one thing is certain in this situation... if ALL of those codes are the result of the same problem, it sure as hell isn't the dash switch.


Take a good look at all of the sensors, actuators, etc on the drivetrain. make sure all of the plugs are in good shape and making contact and follow the wiring to make sure one of the harnesses didn't get cut/damaged.

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