The thermostat is electronic. When it shorts I believe it fails to wide open so that you don't overheat. It will cause long warm ups and excessive emissions. That is why you have the SES light on. It will need to be replaced. The thermostat runs about $100. An Indy would probably charge around $250 to install it. Belt and tensioner are recommended for replacement together about every 4 years. The probably last twice that long. I wouldn't worry about the tensioner until it starts making noise.