Build-plate temperature is lower than 0°C. It happens when build-plate temperature sensor is broken or its cable is broken.
a. Re-plug J12 in main board, re-plug J3, J4 in power & UPS board. Check if there are damages on the cable.
b. Unplug J12 in main board, measure the resistance via pin 4&5, it should be around 100KΩ @ 25°C indoor temperature (Figure 03-1). If the resistance is higher than 300KΩ (equals to 1°C), the temperature sensor or its cable have issues. Replace the sensor and cable as needed.