Mini Split Zone Not Working

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One of our Diakin mini-split fan handlers quit working. A diagnostic scan revealed it was a COMM error in the fan. We purchased a new circuit card for the fan, installed it and it is completely dead now! The old board still remains "half dead" (flashes error but doesn't operate). Is there something else that can be checked on this unit before I go out and purchase another (non-returnable) circuit card. That seems like an expensive path to "troubleshooting".

 

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...one thought I had was to swap a circuit-card from one of my other fan units, to try to isolate the exact offending device.