Does the schematic work?

[j9brown]

I'm in the process of figuring out a similar control system for my van so I'm looking at what others have done.

I noted in your schematic that you are using BJTs to simulate button presses. Each transistor's emitter is tied to GND and that has the effect of connecting Pin 7 and Pin 8 of the RJ45 socket together. I think that shorting these pins will interfere with how the Maxx Air fan's microcontroller scans the buttons and it could potentially damage the equipment. Does it work?

For my own purposes, I'm thinking about using optocouplers to simulate button presses so my control system remains electrically decoupled from the fan itself. I'm also thinking about using more optocouplers to passively monitor the state of the buttons on the key pad as the Maxx Air microcontroller scans them to help maintain state synchronization.

(That said, I'm awfully tempted to simply replace the built-in circuit board...)