13 C-Max Energi

My '13 C-Max Energi had a similar problem and it was due to the audio control module computer staying in an "on" setting and draining the 12 V battery. You can charge the 12 V battery - using the remote connections under the hood makes the job fairly easy. In our case, we had been experiencing the radio getting stuck on (even with the car turned off) and I had to pull the fuse briefly to reset it. That's on the same circuit as the audio control module. Pulling that fuse out also stopped the drain on the 12 V battery but it's a temporary fix since the hazard warning lights are disabled unless you put the fuse back in. We then took it to the Ford dealer who's mechanic reset the audio control module. No idea what that means, but they didn't charge us anything and the problem has been gone since then, nearly 2 years ago.

I recently had a similar problem on my 2013 C-Max Energi. After driving on battery only, the trip mileage would read anywhere from 80 to 150 MPGe. Then after some work at the dealer, it started reading 999.9. The answer is in the menu system. Using the arrows on the left of the steering wheel, left arrow to arrive at the main menu. Down arrow and select: Settings, Display, Units, Fuel Economy, Fuel + Electric. Somehow mine had gotten set to Fuel only. This fixed it for me.