Frigidaire Freezer is same temperature as the refrigerator

Recently my Frigidaire side by side freezer has warmed up to the same temperature as the refrigerator side. I checked for frozen evaporator coils which had a little frost but wasn't frozen solid. The evaporator fan is running and so is the condenser fan. I cleaned out the condenser coils and the compressor is running. I've noticed that the freezer temp reads -6 deg F at times but a thermometer I placed in the freezer is still reading 32-36 deg F. Would a bad freezer thermistor cause the freezer to warm up, or could it be the controller? Any thing I could troubleshoot with my multimeter? Thanks. Model PSHL69EGSS1