I got "Service High Voltage Charging System" last Monday while I drove about 10-15 miles in engine power mode. I could not charge my Volt after that message showed. After service, I was told it was caused by low coolant. Since that was the only problem they found. Not quite sure how it was related. Two days after I got the volt back, the same message showed up again after 20 mile drive in gas today. Does anyone have the same problem? They seems have no clue what causes it. Thanks.