Troubleshooting a heat pump defrost control board requires checking power inputs, sensor resistance values, and test pin signals to determine if the board is failing to initiate defrost cycles. First, turn off the power to the outdoor unit and inspect the control board for physical signs of damage, such as burnt relay contacts, bulging capacitors, or loose wiring connections. Next, restore power and measure the low-voltage 24V supply across the R and C terminals to ensure the board is receiving steady power from the indoor thermostat. To test whether the board itself can initiate defrost, locate the test pins (usually labeled TEST or TFS) while the system is running in heating mode with a call for heat present, and temporarily bridge them with an insulated jumper wire to force a defrost cycle. You can verify that the board is functional if bridging the pins successfully switches the system into defrost mode, stopping the outdoor fan and shifting the reversing valve.
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