Open the bitmap. Pin 21 is connected to resistor R5842. R5842 is a safety resistor, so the two ends are connected. We need to measure the resistor to see if it is damaged.

Attach the motherboard to the holder. Heat with Hot Air Gun at 260 °C and airflow 2 to remove the black adhesive.

Measure with the multimeter. The resistor is found to be good. It can be judged that the fault is not caused by the resistor. The fault should be in the circuit of the middle layer.