Heater blows out cold air this started 2 days ago.Is it the thermostat. on 1999 Ford Windstar

This problem just started a couple days ago.

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3 answers
If it don't run hot,it's not the thermostat.Check the water level in the radiator and if that don't work then the heater core is stopped up.
A leak any where in the coolant system could cause an air pocket in the heatern core.
The fan is broken