The heater control valve may be defective. Look in the engine compartment for coolant hoses entering the firewall; one of these should have the valve on it. The heater core may also be partially plugged. You can attempt to flush this using a hose at one of the core entry hoses.
When you find the valve you may be able to watch its operation. Thus use the heater controls and see if the valve arm moves when it is supposed to.
If this is an automatic climate control system there may be a thermostat to regulate temp by opening and closing the valve. Or the heater controls could be defective.
But start with the heater control valve.
When you find the valve you may be able to watch its operation. Thus use the heater controls and see if the valve arm moves when it is supposed to.
If this is an automatic climate control system there may be a thermostat to regulate temp by opening and closing the valve. Or the heater controls could be defective.
But start with the heater control valve.
