Whirlpool American Fridge Freezer?

Hi there, I have the following Whirlpool American fridge/freezer 858616815010 (S25D RSS33). Had chattering issues which are now sorted (replaced capacitors as recommended on another forum), but the freezer elements keep getting completely iced up and the display indicates that the temperature is not cool enough (instead of -18 on display it shows --). Do you know what this is likely to be? If so, what are the parts I require?

Thanks in advance and kind regards, Alistair