Short out when plugged in

follow the length of the cord and look for anywhere the insulation has run threw. if you find any spots replace the cord. it is also possible for the block heater to internally short out so if you don't find any chafing put a new block heater in the block heater may come with a new cord if not get a new cord at the same time no point in putting a old cord on a new block heater. neither are terribly expensive or terribly hard to change. sometimes they can be a little seized in and a bit of a pia if your working outside in sub 0 temps but really not that bad of a diy job.