Will you please send me the model number, so I can get information for you?
The C2 error code indicates that the thermister that's located at the evaporator is not operating properly, and it needs to be replaced. According to Norcold, this repair is not customer serviceable. They recommend that you contact the nearest authorized service company. If you find to find it, go to www.thetford.com, and select the 'locate a dealer' button from the list on the right of your screen.
If you don't see your model on the RV list, just choose any unit to get to the dealer locator menu.