this is a issue that is always been a bit of agrey area i suspect we have all been caught by a daikin with acommunication faulght, firstly i always check the incomeing supply then thermistor resistances depending on fault code if invertor i carry out a invertor check then a comp ohms test then check in out supply to each board to sea if each unit are talking to each other if all ok we have made a tester to clip to each board on a mhi and a daikin to test each area however usually i have found board failure due to someother fault ie a fan to earth . then of course we have the cost i was quoted the other day 237 pounds for a outdoor board on a daikin on a 7 year old product when a complete system is available for 350 out of the same manufactuer so damed if you do damed if you dont any ideas fellow engineers?