Most of the time it's the hubs, Just plug the red vacume line right at the vacume selonoid behind the battery. I just put some silocone on a bolt around the same size and plugged it and walla, my heater controlles work again in 4wd. I really like still having shift on the fly but just have to lock my hubs in ahead of time. May bug some people but doesn't bother me so much. Yes, it sort of defeats the purpose of ESOF but I'd rather not keep replacing hub assy. every 50K from living in wisconsin with the salt and crap on the roads(hard on the seals). good luck guys.