A while ago I had a similar issue with my old RX8 that had been stood a while.
It started up fine and the temperature, battery and oil gauges were all fine etc but the EML was on and I couldn't go over 2-3k revs.
I thought it was going to be a pricey fix but it turned out to be my battery at fault. I tried recharging it overnight, put it back on in the morning, started up fine but still wouldn't go past 2-3k revs.
So I decided to order a new battery and swap it out. This resolved the problem straight away.
Not sure if it is going to be the same problem you have though as I didn't have a fault code reader at the time, but thought I'd throw it out there in case you have a spare working battery you can try.