07 4 wd won't engage

my 07 envoy has 39,000 and is out of warranty, the 4 wheel drive does not engage when i turn the switch the lights on the switch flash and the 4 tire chassey with a wrench lights up on the dash (trouble light) any ideas fuse hopefully thanks

Many this can cause this it should be scanned for some info. The bcm has control of engagement from the dash switch and it controls the transfer case encoder. You can check the fuses under the rear seat for the 4wd, I believe.