Glenn I have a few Magnum collas also that don't beep when its cold outside. They kida "click" and lose their range real bad. Ever seen that? Danny

glen how old is the collar. i have a had this happen before could be the switch got stuck while the hound was running. thats prob why when you hit it against the tailgate it started to work. call the magnum guys if you keep having a problem. if the collar isn't that old and under warranty they should take care of it.