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Note the cable position at valve at fully opened and fully closed position.
Disconnect the cable and check the valve operations manually.
Operate the valve manually and compare the fully opened and closed position. If there are any difference, readjust the cable and retest.
If no difference, fullly open the valve and with engine running, compare the heat of both hoses, they should be similar if coolant is flowing through.
Did you recheck the coolant level after the thermostat replacement? Any air trapped in system can cause the heater to work erratically.
Another possible cause would be the air mix door.
To test if the heater core is clogged, disconnect both hoses and use a water hose to flush through. If water flows freely, it should be good.
Monday, January 4th, 2010 AT 7:20 AM