How do i know wether the thermistor is shorted / open, defective?

If this device "it has three terminals on it", I believe this part is not a thermistor, rather a power resistor (2 inside part)...does the part show a small diagram with resistor symbols (jagged lines) printed on it? Does it have a part number? Also attempt to measure the current at the fuse...first determine if it is AC or DC and if the projected value may blow the ammeter's fuse, then attach the leads of an ammeter at the fuse points (watch polarity if DC!) and set to highest range and progress to where lowest possible range can be used to get a decent measurement. Does it come close to the fuse rating or far above it?

Hello clydex, Yes you can use an ohmmeter, however you will need a hot air gun to. Let me explain: There are two types of Thermistors +Temperature coefficient and - Temperature coefficient. The +temperature thermistor reacts this way: as the temperature goes up so does the resistance of the device this is used when dealing with degaussing coils as the current passes through the coil it heats up the thermistor until the thermistor cuts off the current needed to run the coil. The -temperature thermistor reacts this way: as the temperature goes up the resistance decreases, this can be seen when dealing with circuits that are designed to short out when excessive current is present. The excessive current causes the thermistor to heat up and the the resistance drops causing the device to short out. Here is how to do it.: connect the device to the ohmmeter and use the heat gun to heat the component up the value of resistance should rise or decrease.

I hope this helps, Thank you, Shuttle83