My Frigidaire model# FPHB2899LF6 is about 9 years old, the other day the freezer temp started going up and has been hovering around 14-17 F, but the fridge section is fine (36-37 F). Everything else seems to be working, the compressor is running, the evaporator fan is running, water and ice maker are also working. Every time the fridge goes to the automatic defrost cycle the freezer temp would decrease to 0 F, after the fridge restarts it would slowly go back up, takes about an hour then it would hover around 14-17 F. I checked the evaporator coil in the freezer and it's completely covered with heavy frost. I manually put the refrigerator in the defrost mode (pressing the up/down arrow on the control panel at the same time for 5 seconds) but no heat came on, didn't melt the frost. Used multimeter to test the continuity on both the heating element and the defrost thermostat and both are fine. What can be wrong? Could it be the control board? There is no error code on the control panel. How would I know for sure (don't want to replace it if it's not, it's quite expensive). Thank you