It could be some sort of "prevent dangerous extrude" where it checks for a certain temperature before letting the extruder motor move. Because if you try extrude with no heat, you could damage the extruder.
In place of extruder thermistor, hook up a test resistor in range of 100 ohms or so. You might need to test different resistor values, in such a way that it will report a temperature around 200C. Or to have a better idea, you can check thermistor datasheet to see which resistance it ought to have at ~200C. Then you can test your extruder motor, hopefully it should work.
In place of extruder thermistor, hook up a test resistor in range of 100 ohms or so. You might need to test different resistor values, in such a way that it will report a temperature around 200C. Or to have a better idea, you can check thermistor datasheet to see which resistance it ought to have at ~200C. Then you can test your extruder motor, hopefully it should work.