I've never done any reverse engineering before, but spurred on by this recent Hackaday article, and this article I found I thought I'd give is a crack.

The first issue: I had no idea what the model number was - it's not written on the unit, nor on the instruction manual. So I just googled for Fujitsu airconditioner remote, hit image results and looked for one that looked the same. Once I found it and clicked through to the source page, I found out that it is a UTB-YUB/GUB/TUB (There are three model numbers depending on where in the world you are).

I found a supplier on ebay (who was actually Melbourne based), who had a new remote unit for $60, which I bought as I wanted a test unit on my workbench - mainly because trying to test things using the unit on the wall would be really annoying.

While I waited for it to arrive, I continued googling to find as much info as I could about it. Thankfully, a number of airconditioning repair places have their installation manuals online. Reading though the them, it was clear there was a three wires that connect the remote to the outdoor unit - +12V, GND and a signal.

Bingo.

Now, I need to work out what this signal wire does.

My first hypothesis was the remote unit worked a lot like an IR remote - every button press sent the complete state to the outdoor unit. If this was the case, it should just be a matter of hooking up a DSO (I have the LabNation SmartScope), copying the signal and replaying it via a microcontroller.

While I could pull the one off the wall, I patiently waited for my test unit to arrive.