Heater is only blowing cold air?

If your 1999 Mercury Sable's heater is only blowing cold air even after replacing the thermostat and flushing the heater core, there are a few other components that may be causing the issue. One possibility is a faulty heater control valve, which regulates the flow of hot coolant to the heater core. The heater control valve is usually located in the engine bay, near the firewall, and is connected to the heater hoses.

Here are some steps you can take to troubleshoot the problem:

Locate the Heater Control Valve: As you mentioned, the heater control valve is not easily visible, but it is typically located near the firewall, close to the heater core. You may need to consult the vehicle's service manual or an online repair guide to find its exact location for your specific model.

Check the Heater Control Valve Operation: Once you locate the heater control valve, check its operation. With the engine at operating temperature and the heater set to the hottest setting, feel the heater hoses connected to the valve. If both hoses are hot, the valve is open and allowing hot coolant to flow through the heater core. If one hose is hot and the other is cold, the valve may be stuck closed, preventing hot coolant from entering the heater core.

Test the Heater Control Valve Electrically (If Applicable): Some vehicles have electronically controlled heater control valves. In such cases, you may need to check the electrical connections and signals to ensure the valve is functioning correctly.

Check the Blend Door: The blend door inside the HVAC system directs the airflow through either the heater core or the air conditioning evaporator. If the blend door is stuck or not functioning correctly, it may be directing the cold air instead of hot air to the cabin. Check for any signs of blend door issues, and ensure it moves freely.

Inspect the Cooling System: Make sure the cooling system is functioning correctly. Check for proper coolant level and any signs of air pockets in the system, as these can affect the heater's performance.

Check for Airlock in the Heater Core: After flushing the heater core, there might still be an airlock trapped in the heater core, preventing the flow of hot coolant. Try bleeding the air from the heater core by running the engine with the radiator cap off and the heater set to the hottest setting. Be cautious while performing this procedure, as the engine coolant can get hot.