Samsing RB1944SL fridge keeps icing up – defroster no work

i have about a 3 year old samsung fridge with freezer on bottom. freezer works fine. fridge ices up about every five days and i have to oremove the panel and melt the ice on the coils so it works again. defrost heater isn’t working properly. i’ve heard the thermistor, thermal fuse and defrost sensor may be the problem. i ran a diagnostics check and it said no sensors were out. i need help and don’t really want to spend a bundle on a repair man when it’s a $15 part. thanks in advance for any help