A heat pump that stays stuck in defrost mode indefinitely—instead of returning to normal heating after ten to fifteen minutes—is usually experiencing a malfunctioning defrost control board, a stuck reversing valve, or a faulty outdoor temperature sensor. During a normal defrost cycle, the control board reverses the refrigeration flow and shuts off the outdoor fan to melt ice. If the defrost control board fails internally or its timer relay sticks, it cannot signal the system to switch back to heating mode. Similarly, if the defrost sensor remains shorted out or fails to register that the coil has warmed up, the board assumes ice is still present and keeps the system locked in defrost. Furthermore, a mechanically stuck reversing valve can trap the system in the wrong operating mode. You can verify the fault by checking if the outdoor fan stays off and supply air remains cool. Professional service is required.
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