Forcing a defrost cycle on an outdoor heat pump is a diagnostic procedure used by technicians to test whether the defrost control board and reversing valve are functioning properly when ice builds up on the coils. To force a defrost, you must locate the defrost control board inside the outdoor condensing unit and identify the test pins—typically labeled "TEST" or "TFS"—while the system is actively running in heating mode with a call for heat present. Using an insulated jumper wire or a metal screwdriver, you briefly bridge or short these two test pins together for a few seconds according to the manufacturer's wiring diagram. This action bypasses the normal time delay and forces the control board to switch the system into defrost mode, reversing the refrigeration cycle to melt ice. You can verify success when the outdoor fan stops and steam rises from the unit.
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