Yes, I like the zero-article option tooPossible but unlikely, Vic.
I'd say the main alternative to the original would be "I put myself in energy-saving mode" - no article.
(That would, incidentally, imply that there was only one energy-saving mode.)
IS that assertion based on what has gone before in the series/episode? It could be that the user is being told something not previously known - there is a mode that is energy-saving and I use it when XYZ"While in the OP context, there must be several already-established and known to the user and robot modes.
After reading about it here, I watched season 1 episodes 1-4 yesterday. They are on youtube, and free. It is quite interesting, with several story lines, each given some time in each episode.(Is the series worth watching??)