I have an 07 Sierra classic and the 4wd is not working. I press the auto 4wd, 4wd hi, or 4wd low and the light just flashes. I can hear the transfer case trying to engage. I have verified the ground connection under the driver and have replaced the dash switch. I do get a "Service 4wd" on the alerts but no codes. It worked fine the last time I used it about 6 months ago. I also performed a fluid change on the front and rear Differentials and a fluid change on the transfer case.