Normally, there are three kinds of possibilities.
1. The temperature shows on the screen is 0 or smaller than the normal temperature, it indicates that the thermistor is broken, a replacement needed.
2. The temperature is much higher than the normal temperature, such as 350 degree, it refers that the thermistor is short circuited.
3. The broken mainboard will also cause such trouble.
4. And of course, these problem exist in each type of machine.