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First thing to do is to make sure that the coolant temperature is up to operating temperature, about 190 degrees F. That is controlled by the engine thermostat. A quick check by feeling the radiator hoses would tell you if it is hot. Using a thermometer taped to the upper hose of the radiator will tell you exactly where you are. If it isn't hot enough, change the thermostat. While you are in the vicinity of the radiator, make sure the coolant level is correct. A low coolant level will reduce the flow to the heater core.
Next, find the two hoses that go into the firewall. Feel them. Are they hot? If not, check for a control device in one of the lines. It may have a vacuum hose attached or a pull cable hooked onto a lever. Work the control inside the car and see if there is any response. If there is a vacuum line on it, pull it off and see if there is vacuum there. If not find the vacuum source and fix it. The problem is normally just a hose pulled off a small fitting. When in doubt, remove the device and replace it with a piece of pipe. Then see if the hoses get hot. If they do, replace or repair the control.
Another reason for the hoses to not be hot is that the heater core is plugged. Remove both hoses and flush the core with a garden hose. Careful because the garden hose can supply up to 100 PSI which can rupture a heater core. Just supply sufficient water to flush out any junk in the core. If the core is plugged too solidly, replace it.
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