dapperdan is fully correct.
any of the new diesels will start at even below zero (F) temps without being plugged in.
But plugging them in during those extremely cold days is a good thing.
Hey, for those in this thread looking for the block heater cord, please do make sure that cord is actually attached to the block heater.
I haven't seen this often, but I did in fact once own a pickup in extremely cold North Dakota. Plugging it in every night, finally one morning at about -45 F it just would not start.
My Dad helped me out, gave me a ride to work, then he fiddled around with my truck. He followed the cord/plug in back from the grille area and found the other end wrapped around some part of the motor. It was not even connected to the heating element!!!!!!!!! And that truck had been starting at temps of -30 F or worse for weeks. Oh, BTW, that was a January in North Dakota where the actual warmest day was -27 F. Heat wave!
Plugged it in for an hour and it took right off. This was a gas BTW, not a diesel. No chance for starting had it been a diesel at those temps.
So heads up, check to make sure ALL parts of that cord are attached properly.