H shows on my fridge temperature ?

Hi there, I have the fridge/freezer APM 6855 and is now showing an "H" for fridge temperature. I cannot set temperature for fridge or freezer, though freezer is working ok. It's been suggested on other sites that the thermostat might be too hot and need replaced. If this is the case would you have any idea how much it would cost or best getting a new one.