I replaced defrost heaters and defrost contoller on GE TFH22PR and the heaters still don't work. One of the old heaters actually shorted out - is it likely the oveload needs to be replaced too? is there somew way to test the system?

you need to have the frig defrost the passage is in the back going from the frig to the freezer so if it is frosted in the back than the air is not moving correctly

thermistor develops a control signal, the heater current does not flow through it. Sounds like defrost heater stays on too long
video here https://www.youtube.com/watch?v=BrSQz-5F1hs

Try replacing your defrost timer it may not be working proper also if it has the coiled heaters behind the insulation make sure they are coming on as well they do burn out.Check heaters voltage and continuity first then defrost timer. If no heated defrost its defrost timer

sizzlling sound most likely from defrost heater

The auto defrost cycle uses 4 components. A timer, a defrost heater, a defrost termination switch and a fusible link.
Any one of those could stop your fridge from defrosting.
The best thing to do is do a diagnostic on the defrost components to work out which component is actually faulty.

You are correct. The defrost thermostat (bi-metal) on these models should only read closed when cold. It should have no continuity when it is warmed above about 55 degrees. They also usually do not fail on these models.
Now if you have replaced the heater, the board and the bi-metal the only thing you have left to do to solve your issues is to replace the evaporator thermistor.
This is located top left on the evaporator coils and is actually what communicates to the control board the status of your coils. The part is relatively in expensive also. WR50X10027 is the part number.
Also Make sure you got part number WR50x10054 for your bi metal.
Good luck!!

Look for frost on rear wall of freezer. Yes? Unplug defrost and remove rear panel in freezer and use a hair dryer to speed thing up. If the defrost heater is burned out replace both defrost heater and defrost thermostat. I have been wrong I'm not perfect but that is my first guess. tell me if the model number is different than the one above your post and I will help you solve this, Thanks, Sea Breeze

The most common problem for your model refrig , to stop cooling the ref section , is a shorted defrost heater in the freezer section . Do you see a light layer of frost on the rear inside lower freezer wall ? Do you hear the fan in the freezer working ? If you have the frost , you will need to remove the rear inside freezer panel ( 2 screws ) , and at the bottom of the coils , is the defrost heater . Part number WR51X10101 . This replacement heater , comes with 2 heaters , due to the old 1 heater burning out so often . The part , sometimes comes with a wire extension , since the blue wire will have to be moved over to the right , with the orange wire , to make it long enough to fit , and sometimes doesn't come with the extension , in which case , you will have to make one . It's also best , to replace the defrost thermostat ( bimetel ) while the panel is off also . Part number WR50X10068 .

it could be that too much ice is forming for the heater to cope with. check the door seals

Check to see if there is any frost buildup in the back wall of the freezer. If so, it is not defrosting as it should.

• possible causes:

1. bad defrost timer
2. bad defrost heater (in freezer)
3. bad thermo switch in freezer

If you don’t know what these are then unplug the machine and let the ice melt in the freezer overnight with the door open. Then try it the next day, if is cooling fine then you need to have a service person replace one of the above items.