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Replacement parts for General Electric TBG16DAZBRWH Refrigerator | AppliancePartsPros.com

First remove the evaporator cover in the freezer so you can see the coils.
Do not let them de-ice.

Manually force a defrost cycle by turning the defrost timer cam (Item 257 in Section 3) till the fans and compressor turn off.
There is usually a hole in the console cover to let you do this.
Now check the defrost heater to see if it is on.
Be careful you do not want to burn your fingers.
If the heater is on then the timer needs replacing, re: it is probably stalling during it's rotation so is never getting into a defrost cycle.
Note: That the defrost thermostat must be frozen for this to work. It opens just above freezing. So if you have defrosted the freezer you will have to hold the face of the defrost thermostat on an ice cube.
Be very careful you do not want to get a shock.

If the heater does not come on.
Remove one wire to the defrost thermostat and measure it, should be 0 ohms when frozen.

If both the heater and defrost thermostat are OK then odds are the defrost timer contacts are toast.
Best way to test this is a live test to see if you have 120 volts across the heater/defrost thermostat combo.