1. Please loose the inner screw from the heating block which fixed the thermistor of the extruder.
2. The possible reason is the extruder thermistor is broken.
When you test the extruder thermistor, just connect one side to A13 port of mainboard, the other side do not connect to the extruder:
If the LCD display shows indoors temperature, the thermistor is good, A13 port is broken;
If the LCD display shows not indoors temperature, the thermistor is broken.