A bad defrost sensor—also known as the outdoor coil temperature sensor or defrost thermostat—presents several distinct operational symptoms when it fails. One of the most common signs is that your outdoor heat pump unit becomes completely encased in a thick, solid block of persistent ice during winter, because a failed sensor cannot signal the defrost board that frost has accumulated. Conversely, a faulty sensor can cause the heat pump to enter endless, unnecessary defrost cycles every thirty minutes even when the outdoor coils are completely clean and free of ice, resulting in lukewarm indoor temperatures and high electricity bills. You can verify a bad sensor by measuring its electrical resistance with a digital multimeter and comparing the readings against manufacturer temperature-resistance specification charts.
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