Brand new here so not sure if i am posting in the right place. Have a totally stock C5. Today the temp gauge spiked to 260. Popped the hood, everything felt normal. Left it shut off for a while. Key on and the dash readout is about 85 degrees Celsius. Start it up and gauge reads 200 for a few seconds and spikes again. And the dash readout says "xxx degrees". Coolant temperature sensor gone bad? Thanks.