Heat is usually a symptom pointing a short on the board. This can either be caused by a bad/damaged part or liquid damage. I would open the phone and confirm that there is not liquid damage present. You can try cleaning any corrosion or liquid damage you see with a brush and 90% or higher Isopropyl alcohol. If this is not present or does not resolve, a diagnostic process that we use is to power on the device with a different flex cable disconnected each time. When the heat goes away, you have identified the cause of the short and know which flex cable to replace.