I've been doing alot of research threw the archives the past two days and have found no posts on this code. I've been getting the dreaded 4wd service light off and on for the last year and half, wasn't really to worried about it do to the fact the four wheel drive would lock in when ever I pushed the button. Well woudn't you know it I went to test it three days ago and it would not go into four wheel drive. I see on my 04GMC Sierra 1500 its most likely one on three things, the switch inside the cab, the 4X4 actuator or the encoder motor(Transfer Case Actuator), or from the code I'm getting a bad ground wire on the frame rail. This is a pain in the A--,has anyone else had this code? I ran a scan tool to pick up this code. I can go into four low, I can hear the motor engage, it sounds kinda funny after it locks in but am unable to get it to go into four high, it just keeps blinking at me. Please advise. About a hour earlier it did go into four high, but after I pushed 2wd button again I cound not get it to go back into 4high. I meant code CO379 not CO327.

 

All fixed, bad ground under the driverside door next to the frame rail, removed the bolts, cleaned the mating surfaces real good and everything works again. Great to answer your own question.

 

I have a 2004 GMC Envoy and getting the problem you mentioned with code co379. Looked around the vehicle and saw the ground under the driver door to the frame and a few under the hood. Did you take a wheel to clean the rail and remount the ground to the pre-threaded hole. If everything looks good, can the code be cleared with the reader or what else can I look for or needs to be done.