I've had many a neutral prong fail on my block heater cords. Then it trips the GFI breaker on the outlet, and will immediately trip it everytime after that. You can meter out the heater through the cord. If you knew the wattage of the heater element at 110v you can calculate the resistance to expect when you ohm out the 2 prongs of the element. But as long as its not totally open, its usually good to go. Ohm the neutral to each leg separately, it should be infinity (open) to each. In my personal experience, it usually been the cord neutral that shorts to one leg or the other.
Replace the cord.
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