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Replacement parts for Roper RT21LMXKQ00 REFRIGERATOR | AppliancePartsPros.com

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 by turning the defrost timer cam (Item 4 in Section 4) till the fans and compressor turn off.
There is usually a hole in the cover to let you do this without taking anything apart.
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.

If not on.
Unplug the unit.
Remove the wire for one side of the heating element (Item 6 in Section 6) from the wiring and measure it for continuity, usually around 20 ohms or so.

If the heater is OK
Remove one wire to the defrost thermostat (Item 8 in Section 6) 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 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.