I have a recurring fault on my volt. The car charges and then I get an error on the dash saying Service high voltage charging system. I have spoken to my local volt dealership (200 miles away) and they said it could be the battery coolant level. I have checked this and it's where it should be. They also said it could be related to the charge port door but I don't see how it could be. They have had the car a few times before and each time just reset it and gave me it back but it keeps happening. I am able to clear the code myself and charge it but it's started happening daily so I know something is more serious now. Any ideas? I was thinking it could be the sensor for the coolant level but that's just a guess. I'm wary that the dealer will just reset it again and do nothing.