If your heat pump refuses to run or locks out when outdoor temperatures drop significantly, it is usually experiencing a safety lockout, severe coil icing, or a thermostat configuration issue. When outdoor temperatures plunge well below the system's design limits, safety controls may prevent the compressor from starting to avoid severe mechanical damage from thick ice buildup or excessive head pressures. Additionally, a malfunctioning defrost sensor or a stuck reversing valve can trap the system in an iced-over state, triggering a safety cutout. Furthermore, if your thermostat is misconfigured or low-voltage wiring is faulty, the system will fail to engage. You can troubleshoot by checking your circuit breakers, inspecting outdoor coils for heavy ice, and switching to emergency heat. You can verify resolution when the system restarts.
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