1. Open the Y box, pull and reset the cable of heating bed.
2. Test the resistance value of heating bed wire.
The two white lines in the middle are hot bed thermistor wires, the normal range is 80-120kohms.
The 1 / 5 or 2 / 6 are hotend line of hot bed, the normal range is 2 / 10 ohms;
3.If the resistance value is not within the normal range, there must be a problem on the hot bed, update the newest firmware for your machine.
4. All that mentioned above don't work, the motherboard need to be replaced.