Determining whether a heat pump defrost control board is bad involves observing system malfunctions and performing specific electrical and diagnostic tests. If your heat pump either freezes up into a solid block of ice without ever initiating a defrost cycle or remains permanently locked in defrost mode blowing cool air indoors, the defrost board is a prime suspect. To test the board, turn off power and inspect it for visual signs of damage like burnt relays or bulging capacitors. Restore power and check for steady 24V input across the R and C terminals. Finally, short the test pins temporarily; if the board fails to respond or switch modes, it is defective. You can verify the diagnosis when replacing the failed board restores proper defrost operations.
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