FF40P Hotpoint fridge freezer not self defrosting

Hi Folks, got a call from a customer with a Hotpoint FF40P, upon investigation it appears that the freezer is not self defrosting a previous engineer had fitted a thermister then would not come back when the fault reoccured. I wired up the defrost stats and element straight to the mains and it defrosted fine. I ordered and fitted a control module and three weeks later I get a call from the customer and the it is exactly the same i.e. not self defrosting. is the a test function on this appliance ? any ideas anybody. Thanks Neil