replacement auto defrost sensor for my amcor hd320 dehumidifier?

where can I purchase a replacement auto defrost sensor for my amcor hd320 dehumidifier.
the units fan runs and the compressor, but dose not go to auto defrost mod. the filter is clean, the water tank is empty the unit just keeps going untill its solid with ice after a few hours witch it never used to, it was very good at removing almost a tank of water a day can you help. Graham