almost all if these I have ran into (and this is about one of the 20 most common error codes these units pop up with) were either the relays (far most common), the connections to the relay box or the box itself not holding the connectors securely, a few were the dash itself.
It may be possible it is the controller itself, or a fuse, (hopefully you have checked all the fuses for a voltage drop first).
when you say the relays "checked out" can you describe further how you tested the relays? have you tried replacing all the relays from a "known good" unit?