Samsung srs2026cvw

Hi,I have a Samsung srs2026cvw side by side american fridge freezer.The problem is it will run for approx 4 hours (after switching off and on) then when using display diagnostics it is demanding defrost cycle of the fridge compartment.It will not come out of Defrost cycle until power is reset again(power permanteley to fridge defrost heaters).The fridge compartment thermal fuse is open circuit due to heaters being applied.(i have temp fitted new thermal fuse to make circuit just in case this was causing no 4 to be displayed on start up)I have checked fridge and freezer sensors resistances and voltages back to the PCB under the appliance.Voltages/resistance are correct to Samsung manual.
The only thing i have noticed is when power is reapplied a number 4 is displayed for 3 seconds on right hand symbol on fridge display then appliance fires up as normal.
I dont want to spend money on a pcb unless 100{e5d1b7155a01ef1f3b9c9968eaba33524ee81600d00d4be2b4d93ac2e58cec2d} but it is looking like a possibility.Also panel on front of freezer door could this cause such a fault?.
Any advice would be most appreciated.Thanks