5 year old BMV712 has worked flawlessly, reconnected to batteries, now showing incorrect voltage and amperage. Voltage bounces back and forth between 6+ and 0. Amperage reads -200+. No change in wiring. Replaced 1A fuse with new one. Checked all wiring connections. Unplugged monitor and reconnected. Tried connecting to 1 battery but same readings. Checked battery with multimeter and between battery and fuse connection on shunt - reading 12.8. Any suggestions? Do I need to replace the shunt, or the whole system?