Disconnect battery first. Then go through all the connections, disconnect, and reconnect, inspecting for corrosion. There have been pictures and reports of people getting water or coolant in there, and causing all sorts of problems. Anything looks bad, clean it up, the best you can with electrical contact cleaner and a nylon toothbrush...flush with more contact cleaner, then canned air.
AND, if there is water intrusion, figure out how it got in there, and how to prevent it in the future. I think sometimes the previous tech forgot to seal things up properly....
Factory M-Sport package with factory installed M-tech bumpers, factory Bluetooth, factory M-Audio 10" dual voice coil subs, Homelink, M5 rear stabilizer bar, Bilstein B14 PSS coil overs, Stop Tech ss brake lines, Akebono Euro Ceramic brake pads, oem Brembo and Zimmerman rotors, Dinan Stage 1 software, Strong Strut front upper stress bar.