I'm assuming the '99 S70 is same as my '98, the sensor would be inside the coolant reservoir and the electrical connector is underneath the reservoir. Detach the reservoir and rinse it off to clear any debris that might be preventing the float from function properly. As you are reconnecting the hoses back, be careful not to over tighten the hose as you can easily crack the plastic. Run the car at its normal operating temperature and check to make sure you do not have a leak.