This has happened to me.

Strangely it was just after the dealer serviced the car. After driving off the lot I got the reduced power warning. I pulled back into the lot and SA had a tech come check it out. Apparently they did not securely reconnect some wiring. I don't really know which ones.

So before you go replacing things, I would make sure that all the wiring harnesses are plugged in tight and securely attached.

Good luck.