How low should the temperature go before I plug this thing in?? 2003 2500 <br>Thanks

 

I plug mine in when the temp outside is going to be about zero. You can plug it in anytime you want and it will start much much easier. I also have mine on a timer so it will start getting power to the heater about 3am and when I start it about 7am it starts right up even at minus fifteen.<br>Tom

 

I plug mine in when temps are going to 20 or below. Some times, higher but it depends on what you want.

 

I would say below freezing. Just in case there is any water anywhere.... fuel, bad coolant mix,etc.