It is a good idea to check the heating cables and thermistor wires of that extruder you took apart:
- are the cables still in place with electrical contact to heater / thermistor?
- is the insulation of the cables still intact & free of crossovers (short circuits)?
- are the other ends of the cables still plugged in?
- are the cables free of any metal parts (e.g. extruder / blower housing) squeezing them?
If all the above is O.K. then you might want to check the electrical resistance of the thermistor using a multimeter. It is described here.