If the defrost heater assembly is defective the defrost system will not work and frost will continue to accumulate.
Auto defrost freezer not working.
If any of them fails to function the defrost system will not function.
Move on to the next step if this does not resolve the defrosting issue.
A manual defrost freezer does not contain any automatic heating elements to prevent the build up of frost or ice.
Replace the defrost sensor if it shows no continuity at around 0 degrees f.
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.
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.
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.
The automatic defrost should now work if there is no other issues with the defrost system.
You can use volt ohm meter to check the defrost sensor for continuity.
It requires the owner to manually defrost the freezer coils as frost builds up.
Do this with all the sections of the timer till you reach the beginning of the cycle again.
5 check if the defrost timer defrost heater and the thermostat are working correctly.