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Sometimes things simply don't work, despite everything looks good. In such cases it's the best idea to work step by step to find the cause.
Resistors change their resistance with temperature. For fixed value resistors this effect is unwanted. Thermistors take advantage of this effect and do so in a predictable fashion.
A typical thermistor circuitry is shown to the right. About all RepRap controllers use the same design.
Connecting components this way gives a voltage on the signal line which depends on the thermistor's resistance and accordingly, on the thermistor's temperature. If the thermistor is very hot, its resistance is near zero, so the signal's voltage will be close to zero. If the thermistor is very cold resistance is very high, signal's voltage will beclose to the supplied voltage, 3.3 volts (or 5 volts on other controllers). In between is our measurement range.
Before searching for firmware misconfigurations, it's always a good idea to check hardware. A firmware can only report the voltage on the processor's signal pin.
Easiest way to do this is to disconnect the thermistor and measure voltage between both pins. With no thermistor connected, it should be full supply voltage, 3.3 or 5 volts depending on the controller.
This measurement is simple, too. Disconnect the thermistor and measure resistance between both pins. At room temperature this should be around the nominal value of the thermistor, typically 10 kOhms or 100 kOhms. Warming the thermistor by hand should reduce this resistance.
Measuring the thermistor while it's plugged in gives false results.
If resistance is always zero, there's a short in your wiring. If resistance is infinite, wiring is broken.