my 2009 mercury grand marqus
With the engine warmed up and turned off. Check the heater hoses under the hood. Be sure they are both hot. If only one is there is a plug of some kind in the heater core or the hoses. If both are hot then it may be the blend door actuator not functioning. Heater core plugs can be removed with garden hose pressure on a cold engine. Remove both hose from the engine and run water through them till it appears the water if flowing freely. Reconnect and try heater with warmed up engine.
When i mean by a heater core plug, i am talking about a mass of rust, coolant solids and metal the make up a gooey ball that can plug up the heater core. Have flushed a few of the these out over the years.
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