The outdoor defrost control board temperature sensor failed open, preventing the reversing valve from switching into cooling mode to thaw the outdoor coil, while an open sequencer fuse disabled emergency heat strips.
Diagnostic Procedure & Field Restoration:
- Outdoor ambient temperature dropped to 22 degrees Fahrenheit with 90% humidity, creating heavy frost accumulation.
- Defrost thermostat sensor failed to close at 30 degrees, leaving the outdoor coil completely choked with ice.
- Stage-2 electric heat sequencer auxiliary relay had blown an internal 5kW fusible link, leaving only 5kW of heating for a 2,400 sq ft home.
- Replaced the defrost board and sensor, verified 4-way reversing valve shift into cooling mode to melt ice within 8 minutes, and restored full 10kW auxiliary heat strip staging.
Experiencing this exact symptom in Pflugerville?
Technicians carry exact replacement capacitors, contactors, and diagnostic test instruments on service trucks for rapid same-day restoration.
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