The automatic defrost should now work if there is no other issues with the defrost system.
Auto defrost freezer not working.
If the defrost sensor trips because it s not working correctly frost builds up on the evaporator fins eventually making the refrigerator and freezer not cool well.
If the defrost heater assembly is defective the defrost system will not work and frost will continue to accumulate.
A manual defrost freezer does not contain any automatic heating elements to prevent the build up of frost or ice.
It requires the owner to manually defrost the freezer coils as frost builds up.
You can use volt ohm meter to check the defrost sensor for continuity.
Move on to the next step if this does not resolve the defrosting issue.
Do this with all the sections of the timer till you reach the beginning of the cycle again.
The defrost heater assembly turns on several times throughout the day to melt away any frost that may have accumulated on the evaporator coils.
If any of them fails to function the defrost system will not function.
The defrost timer is such that either the compressor or the defrost heater is on but not both at the same time.
If the defrost timer is not advancing replace it.
All you have to do is set it to come on for 15mins then of for 30 mins.
A defrost thermostat opens the heater circuit when the evaporator temperature rises above a preset temperature 40 f 5 c or more thereby preventing excessive heating of the freezer compartment.
A quick fix if the fridge freezer is working ok but the auto defrost is not is to connect it to the mains via a 24hr timer.