I received an urgent after-hours call from a customer whose heat pump was running constantly but blowing cold air, with the outdoor unit completely covered in ice. I diagnosed a failure in the defrost cycle, tracing the issue to a faulty defrost control board that was not signaling the reversing valve. I replaced the bad board with a new one from my truck stock, initiated a manual defrost cycle to clear the ice, and confirmed the system successfully switched to providing powerful heat. The homeowner was extremely grateful for the fast emergency response that restored their heat on a cold night.