What was found
AFT CABIN TEMPERATURE CYCLING BETWEEN 15 DEGREES C AND 40 DEGREES C WITH TEMPERATURE CONTROLLER KNOB IN FULL COLD POSITION. OCCURRED AT FL 360, 340, AND 320. ALL OTHER INDICATIONS NORMAL. SUPPLY DUCT INDICATED 15 DEGREES C - 40 DEGREES C BUT PASSENGER CABIN REMAINED CONSTANT AT 20 DEGREES C. ACFT GROUNDED. NO FAULTS FOUND ON BOTH PACK/ZONE TEMP CONTROLLER BITE CHECKS. FOUND AFT CABIN ZONE TEMP SENSOR FAN WORKING INTERMITTENTLY. REPLACED FAN. INSTALLATION TEST PASSES. GROUND VERIFICATION SUCCESSFUL IAW FIM 21-61/863.