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Replacement parts for Whirlpool ER8YHMXSL00 REFRIGERATOR | AppliancePartsPros.com

See the attachment for the tech sheet.
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Is my next step to replace the defrost thermostat?[/COLOR]
I am not a big fan of plug & chug so I would do the following.

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

If yes.
Manually force a defrost cycle, see the tech sheet. When the fans and compressor turn off, you are in defrost.

I never trust replacement parts 100%. Spent too much time when I first started going in circles only to find that the part I replaced was no good. If possible it is always a good idea to check a part before installing it or at least after it is installed.

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 never getting into a defrost cycle.

If not on.
Unplug the unit.
Remove the wire for one side of the heating element from the wiring and measure it for continuity, usually around 30 ohms or so.

If the heater is OK
Remove one wire to the defrost thermostat and measure it, should be 0 ohms when frozen. Note that it opens just above freezing so must be frozen to check it. Also inspect it, if it is bulged at all replace it even if it measures OK.

If both the above are OK then odds are it is timer but could be a gad wire or connector..
Best way to test this is a live test to see if you have 120 volts across the heater/defrost thermostat combo.
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Also, how do I replace that in the harness? Or should I just buy a new harness? Would a new harness already have the defrost thermostat attached?[/COLOR]
See the defrost thermostat (Item 32 in section 4).
Unplug the unit and cut the wires going to the old thermostat.
Be sure to leave yourself enough wire.
Now connect in the new thermostat using a couple wire nuts.
Keep the wires away from the heater.
Also attach/zip tie them so that the open ends of the wire nuts face down, keeps water out of them.
Some people also like putting some calking in the iopen ends as an extra precaution.