daewoo FRS-U201A1 keeps blocking duct with ice

hi

I’m hoping this will be an easy diagnosis for someone.

compressor runs ok , fan in freezer section runs ok ,BUT after a couple of days freezer section starts to thaw at top of compartment and fridge side is warming up .

removal of the rear panel in freezer section , reveals coils frosted up (not ice packed , looks ok to me) apart from the duct behind the evaporator about 1/3 way up is totally blocked with ice.

once defrosted with hot air gun and restarted it works okay until a day or two later and rinse/repeat.

I have carried out forced defrost to sort the issue and it sometimes clears the blockage sometimes doesn’t – the heater element does work in forced defrost mode.
I have carried out diagnostics where you press the refrig and frz buttons and the system does a self test – no warning are flagged.

I have changed the defrost sensor – no change.

there are 2 other sensors associated with the evaporator , I don’t know how to test those or whether they could be the issue? , they have a small resistor in them by the looks of things , can anyone help me in this regard?

I guess the next thing would be a faulty pcb?

I’m hoping one of the gurus here may have seen this issue on this model before and can point me in the right direction.?

thanks for reading and for any help