2004 Ford Explorer Idol & power problem

Our Ford Explorer acts as if its not getting enough power to the tranny & car functions to work. It has a new heavy duty battery, brand new alternator and a total electrical system check. I was told to have a sensor checked, but don't know what sensor would cause all this. Its to be a sensor on the firewall, so any info to this problem would be greatly appreciated. I was told it regulates power to tranny & fuel emissions. Help!