Samsung 2026cvw american side by side

Hi back again after various tests carried out on my fridge freezer.Original problem was thermal fuse in fridge compartment open circuit.
Unit will only work for approx 4 hrs then permanent demand applied to defrost heater in fridge.
Today i carried out forced defrost in( test mode) fridge section,heater gets hot and completeley defrost evap.At the same time im checking voltage/resistance going across defrost sensor in fridge and it gets as high as plus 20 degrees which is approx 5.970 ohms and 1.869 volts.
The heater remains on until switched off at mains.
I have suspected pcb all along as it is getting correct resistance/voltage back from defrost sensor but still supplies power to heater.
My question is can the panel on front of freezer door give such a fault?.or is the pcb in control of this heater>
Thanks again Rick