SV04 Showing (-15C) on left extruder on startup will not heat

I have changed the entire head, heater cart, thermistor and heater block and it still won’t heat. Continuity on the heater cart between the extruder and the ribbon plug-in is fine, This failure came on slowly then it just quit and I get a thermal warning if I try to heat the extruder. Both my right extruder and the bed show normal room temperature.
Any help would be appreciated.

There are several parts that may possibly cause the negative temp : Motherboard、thermistor、adapter board and ribbon cable.
you’ve changed the thermistor so I assume it is good.
For motherboard, try to directly connect the thermistor with the TB-end(thermistor-bed end) inside the board(I remember that it doesnt have the TH(thermistor-end) one). If the temp doest change, the problem may be from the motherboard. (refer to the pic)
However, if the motherboard doesnt have probelm, it seems that problem is from the adapter board or ribbon cable. In this case, you’d better to get a new adapter part and ribbon cable.