You need to visit the dealer or a good independent shop that can pull the trouble codes for the 4-wheel drive system. The trouble indicator is a sign that the computer has detected an electrical problem related to the 4-wheel drive system. The computer will store a code that will direct the diagnosis of the problem. The shop manual will provide steps to follow for each code.

It is likely to be an electrical problem, so you need to carefully inspect wiring, and make sure the connections are good. Make sure no wiring harnesses are pinched. Beyond that, you will need professional equipment to make the diagnosis.

It's usually the 4x4 switch or the encoder motor,you can try fixing or replacing the switch yourself but if this does not fix it take it to a gm service center to check out the encoder motor. Before spending the $76 + on switch; remove the old one and clean the posts on switch then apply electrical dialectic grease to terminals. The switch resistance can build up due to oxidized contacts, and the computer sees it as a circuit fault. Again, if you scanned the codes, it would probably identify a switch circuit fault but there could be other issues.

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