There is a discussion thread on this topic on the Volt forum. The coolant sensor is prone to fail, and it then triggers this message. There is a remedy, with the WOT sensor, which is for sale on the site for about $40. It functions by removing the ffailure-prone coolant sensor, and replacing it with something that sends a positive signal. (You have to remove the negative 12 volt battery prior to installing it, so it doesn't trip a message and trigger the SHVCS message again) Of course you then have to check the coolant level manually, as the sensor is no longer functioning. I have to confess that I ordered this workaround, but have not yet installed it. At first I did it because I didn't want dealer technicians noticing it and denying me warranty coverage for other items. But now that my warranty is expired, I should install it.