why does lights flash when in 4WDhi or 4WDlo truck does engage but lights flash on 2003 GMC Envoy

both 2wd and a4wd work fine and lights for these 2 selections go solid like they should.is there some sensor that detects when the truck is in 4wd and where is it located. maybe it is defective

Asked by for the 2003 GMC Envoy
1 answer
have codes scanned with a good sacnner not just an obd11 scanner.then post codes so we can adv