Hotpoint FF175BP Fridge freezer defrost cycle not working

[zombietp]

Hi. I have a Hotpoint frost free fridge freezer FF175. Serial no 801093067. Fridge warms up as the defrost cycle isn’t operating- the evaporator and duct get blocked with a build up of ice (mainly powdery). Have tried defrosting but it just does it agin a few weeks later. The fan is operational. I have checked the heating element with multi meter and it isn’t open circuit so I summise that it the heater itself is OK but that the heating cycle isn’t initiating. Connected to the heater is what is described as a thermal cut out connected to the same connection block as the heater. there are also two thermistors/temp sensors -1 at the base of the freezer and 1 tucked into the top of the evaporator.Which part initiates the defrost cycle and which bit to replace is the question. Can you buy the thermal cut out seperately?(Comes with the heater element on hotpoints spares list).If it’s the sensors- do you have to take the back off the unit to replace them?My other line of thought is could it be the thermostat damper which is in the fridge section behind the duct cover plate which controls the initiation in which case I could replace this part The difficulty is sussing out which bit is ng so I can replace the right part and hopefully get it back under control. Suggetsions on how to narrow it down further would be appreciated.

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