The heaters on that printer are either on or off. The thermistor is supposed to tell the printer when the heater has reached the temperature you requested and toggle between on and off to keep the heat consistent. If the thermistor fails the heater will either not heat up properly or heat up too much. Find a way to take the temp of the platen and compare it to the reading on your heater settings on the printer. If they are way off I would go with changing the thermistor first. If they are the same, you don't have a problem.