Hotpoint FF71 frost-free fridge/freezer?

My frost-free fridge/freezer is building up too much ice at the rear of the freezer. So much that it is blocking off the air flow to the fridge, so it's far too warm in the fridge. The fan is working when the fridge door closes. Does anyone know what is the most likely cause of this is?
Richie, March 2005
Which of the three plugs on the rear of the freezer controls what? I have a green, white and yellow plug.

Spike, April 2005
you will need to repalce the thermisters common fault

ian, March 2005
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