I purchased my C-Max Energi new in 2014 and the car worked nearly flawless up until about 50k miles. Every once in a while the 12V battery would go dead for no apparent reason. Now the car is at 94k miles and the 12V battery dies if the car is left off for more than a day and unplugged. I've replaced the 12V battery twice but its not a solution to the problem, only a very temporary repair, as deep discharging batteries kills them.

I've taken it to 2 dealers who said they couldn't reproduce the problem and didn't know how to fix it. I did some investigating myself and found that relay 9, which I believe is for the vacuum pump, cycles after the car is off. I'm not sure if vacuum pump is controlled by the ECM or a separate module but something isn't turning off when the car is keyed off. I've read reports of people replacing the Audio Control Module or the Body Modules for the windows to fix the problem but I tried pulling fuses for both and the problem still persists. I've also found three TSB's which state the TCM, DC/DC, and other components should be re-flashed as a potential fix which I'm going to have my dealer do in the near future.