Kay W Haughton , LA November 01, 2014 Appliance: Frigidaire Freezer MFU20F3GW7 Frigidaire Freezer Mfu20f3gw7: Defrost would stay on long enough to defrost my freezer and every thing in it. How to replace Defrost Thermostat. I replaced the timer and the defrost thermostat, it took about 30 minutes, just got thru so I'm waiting to see how it works before I start filling it back up again, My husband is at work so I did my self. 1 People found this story helpful Do-It-Yourself Rating: Repair Time Estimate: 30-60 minutes