Head gasket. Waterpump. Or clogged radiator. Possibly has an air lock pocket of air in cooling system did u bleed it correctly?
The best indication of a bad thermostat is when the radiator fluid (Antifreeze) boils over from the reserve tank, not the radiator. This is caused by the thermostat not opening at its preset temperature and all the extra antifreeze is pumped directly to the reserve tank. You will also have loss of power and the vehicle will likely stop running soon after the antifreeze boils over.
Thermostat sticking. Replace thermostat and gasket.
Cracked.head
Is you Geo Tracker overheating or the temperature is running almost hot then you need to change you thermostat
The thermostat is stuck open and needs to be replaced.
Check the engine thermostat.
My 08 sierra had this same message. It wasn't hot and the antifreeze reservoir was full. The thermostat wasn't reading correctly. We had that replaced and so far so good.
Thermostat needs to be replaced, it is not opening (most common). Is electric fan on radiator running normally from time to time?
Perhaps plugged radiator - have it boiled out or replaced
Could be your electric fan on the radiator is shot, or the relay is bad.
Antifreeze could be running out often due to a leak. It is important to inspect the radiator for small cracks or smoke.
have you checked the fan