The sensor itself may be defective. I replaced mine and it fixed the problem. There is a magnetic float shaped like a donut that sits around the sensor. If the float moves freely but the low coolant light stays on, try replacing the sensor. It pulls out from the bottom of the reservoir. The sensor is quite cheap ~25 bucks and can be ordered from a Volvo dealer. Make sure to siphon all the coolant out of the reservoir before pulling out the sensor..