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Defrost system failure (frosted evaporator)

The defrost system (heater, defrost thermostat/thermal fuse or sensor, and the timer or control board) must melt the coil frost every few hours. If any of them fails, the coil turns into a block of ice and airflow stops. Test the heater and the limit with the coil still cold.

Single source ~90 min moderateBrands: All

Before you start

  • Unplug before removing the evaporator cover.
  • Do not chip ice off the coil; melt it with gentle heat.
  • Never apply 120 V to a heater outside the unit unless you're trained to; keep it to resistance tests.

Diagnostic steps

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OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

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Defrost system failure (frosted evaporator) · OnCall Tech