My carrier thermostat was working, but then fan ran continuous. Checked thermostat, not lit.
Book says internal power source, not battery, could this have given up, even with power still on? Is this renewable or do I need a new thermostat? It is 10 years old.
Thanks

Unfortunately likely it is the relay for the fan. A cheap little part located on an expensive circuit board and prone to failure. The thermostat would be much cheaper to replace. Try pulling the green wire off either the circuit board or thermostat, and see if the fan still runs. If so, the problem is in the furnace.