I was checking to see why my block heater stopped working and found that the plug had burned off.  It looks like it got moisture in it and corroded and caused it to arc.  Something you may want to check once and a while.  If it was kept clean it would probably still work.  Picking up a new one from Napa today.

Mine did that and thought I would have to go to Cummins but the auto pars store had a whole box full of different cords. They let me find one that fit and it's been good ever since. I think it was 14 bucks or something.

I had to replace both