Replace a Defrost Control Board (Heat Pump)

Match the OEM board, set the defrost interval pins (30/60/90 min) + termination per spec, transfer the defrost thermostat/sensor connections, then verify a forced defrost — that it terminates on temp/time and that it energizes aux heat + the reversing valve correctly.

Approx. 75 min

Before you start

•Board confirmed as the fault: the unit ices up and never defrosts, or defrosts constantly / won't terminate, the defrost LED/diagnostics point to the control, and the defrost thermostat/sensor itself checks out OK.
•Exact OEM replacement board for the heat pump (timing logic and the demand/time-temperature method vary by maker).
•Power can be killed at the disconnect + breaker before opening the control box.

Tools & materials

Multimeter · Nut drivers/screwdrivers · Exact OEM defrost control board · Phone camera + labels (photo wiring + the interval pin position) · Wiring diagram

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Isolate & document

Pull the disconnect, verify 0 V, bleed the cap
Kill power at the disconnect + breaker, verify 0 V at the line terminals, and discharge the run capacitor before opening the control box.
Tools: Multimeter
⚠ Lockout/tagout and verify 0 V before touching the board.
Photograph the wiring and the interval pin setting
Photo every lead and especially the current defrost-interval jumper/pin position before removing anything — line, the reversing-valve solenoid lead, the compressor contactor coil, the aux/emergency heat call, and the defrost thermostat/sensor leads.
Target: All wiring + the interval pin position photographed before removal

Swap & set the timing

Confirm the OEM board matches
Verify the new board is the correct OEM part — the defrost method (time/temperature vs demand) and relay layout must match so the cycle times and terminates correctly.
Target: OEM part/revision and terminal layout match
Set the defrost interval pins and termination
Set the new board's defrost-interval jumper/pins to match the spec for this install (commonly 30, 60, or 90 minutes of accumulated compressor run-time between defrost checks), and set the termination temperature/time per the maker. Wrong interval = ice buildup (too long) or wasteful nuisance defrosts (too short).
Target: Interval pin set to spec (30/60/90 min); termination temp/time per maker
Transfer the defrost thermostat / sensor connections
Land the defrost thermostat (or coil temperature sensor) leads on the new board exactly as before — this is what the board uses to know the outdoor coil is iced and to terminate when it clears. Reconnect the reversing-valve, contactor-coil, and aux-heat leads per your photos.
Target: Defrost sensor/thermostat + RV + contactor + aux leads landed per the original

Force a defrost & verify

Initiate a forced defrost
Use the board's forced-defrost pins/jumper (or short the test pins per the maker) on a call for heat to start a defrost without waiting for the full interval.
Target: Forced defrost starts on demand per the board's test procedure
Confirm the full defrost cycle behaves correctly
During the forced defrost, confirm: the reversing valve shifts to cooling (to send hot gas to the outdoor coil), the outdoor fan stops, and aux/electric heat energizes to temper the cold indoor air. Then confirm it TERMINATES on temperature (coil warms) or the fail-safe time, returns to heating, and the outdoor fan restarts.
Tools: Multimeter
Target: Defrost: RV shifts, outdoor fan off, aux heat on; terminates on temp/time, returns to heat, fan restarts