My 2004 Tahoe has on occasion the service 4wd comes on and no lights on auto 4wd switch. I cleaned ground wires on frame problem still persisted. I then checked ohm values on switch they all were okay except the auto top button. I wasn't convinced so I took to dealer they scanned it came up code co327. They said decoder motor assembly needed replacing. Cost was $1100 and an aftermarket decoder unit was about $360. I brought Tahoe home reached underneath tapped on decoder motor.assembly. Turned on key and the service 4wd light was out and lights back on in switch . Worked good for a few days and when I switched to 4 high the service 4wd came on again. Tapped assembly again and back to normal operation. I left it in Auto and has been working fine. I will be changing encoder but this may work for others if you are in a spot requiring the 4wd mode.