2002 GMC Envoy

Transmission problem
2002 GMC Envoy 6 cyl Four Wheel Drive Automatic 61000 miles

My 2002 envoy fails to shift into 4 wheel drive.
June 5, 2010.

Many things to consider
check fuse 1st
have to scan for codes

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(R).
Try to communicate with the TCCM. If no communication, go to step 2. If communication is possible, go to step 5.
Remove the underhood (ATCM or TREC) TCCM fuse.
Wait 30 seconds.
Reinstall the fuse. The module should " wake up" and communicate with the Tech 2(R).
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.
Reprogram the TCCM with the latest TIS software using normal SPS procedures.
Verify operation and that no codes are present.
IMPORTANT: For the 2002 model year, the TCCM must be replaced and reprogrammed

Jun 5, 2010.