https://www.youtube.com/watch?v=yI5gPk5TirU
https://www.youtube.com/watch?v=kPlTuQJTn_M&t=41s
TH-BED series faults are generally caused by the temperature sensor at the hot bed or the fault of the hot bed. Sometimes it may also be caused by solid damage.
If the problem still cannot be solved after replacing the temperature sensor, you need to replace the heating bed with a new one.