I have 2x AOHA24LAT3 units each running 3 indoor air handlers (ASHA12LACM)

One unit ( all 3 air handlers) has the green 'timer' light blinking 3 times.
Please tell me what this error code means, and what can cause it?

I have changed the entire PCB assembly from the working outdoor unit into the faulty one but this has not solved the problem.
Worse news is that when I changed it back into the original working unit I now have the same error code (3x green) flashing on the previously working units!

Is there anything you can suggest to help?