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One common component to fail on this model here and cause this issue is the 4wd switch itself on the dash>GM had even released a bulletin on the newer body style 1999 (not the classic body style that ran from around 1995 to 1999) to replace this switch if there is a code B2725 set in the transfer case computer.I have seen the encoder motor fail as well on the transfer case and cause this here needing replaced but most of the time it is the switch itself needing replaced.
You will need to have the fault codes read out/checked to see what codes are set but this needs done using a scan tool that will plug in and communicate with all computers in your vehicle.Once this is done any codes set need diagnosed to locate and correct this issue.If there is a code of B2725 set in the transfer case computer the switch needs replaced on the dash,the push button switch.
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