If your coolant overflow tank is full to mark and light is still on then the sensor in the tank has most likely failed. And the only fix it to replace the sensor.
.
Dec 29, 2009 at 2:00 PM
Repair Safety Notice: This information is for general instructional purposes only. Vehicle repair can be dangerous. Verify all information, follow manufacturer service procedures, use proper tools and safety equipment, and consult a qualified repair shop when needed.