1999 V70XC Several months ago, my coolant light came on. It was a surprise, as there had been no sign of a problem previously. I opened the hood and the reservoir was basically empty. No sign of any coolant on the floor. I filled it, but then it seemed that I wasn't getting the normal amount of heat with heater on. Fluid level still fine. Changed out the thermostat and everything seemed better. About a week ago coolant light came on again. Reservoir level fine. OBDII indicates no problem. Temp gauge indicates all is normal.

Faulty sensor? Could the sensor in the reservoir been harmed somehow when the reservoir was empty?