Follow the wires coming from the grip to the plug, unplug and test to make sure you have 12+ volts at the plug, test the element with an OHM meter. As mentioned above it's probably the element.
That machine has a heater element under the grip, as do most Polaris models. Remove the grip, without damage if you can, and wrap the new element around the bar. Element is about $30.00 and grip is about $10.00-$15.00 depending on model.