The center wall should always be warm and sometimes it can get hot, which is normal. This is because one of the hot gas lines from the compressor runs behind the wall. As long as the compressor is running this line, which gets very hot, warms the wall to keep moisture from accumulating on it. This is an energy saver feature, to prevent the use of a heater. If the wall gets excessively hot for a long period of time, it can be caused because the condenser coils are dirty and need cleaning or the condenser fan motor has either quit running or slowed down. One other reason it can get excessively hot is, if it runs more then normal. This can happen due to a large warm load of food placed in it or if the unit is located in a warm (hot) location. In these cases, it will cool down as the load cools or the unit can be moved to a cooler location. If you need more assistance, you can resubmit your question with more details.