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With the vehicle warmed up and running feel both heater hoses with your hands. If one is hot and the other is luke warm or cold, the heater core needs to be flushed by itself. Flushing the whole system doesn't unclog the heater core. Disconnect both heater hoses from the engine, fabricate a fitting to attach a garden hose to one of the hoses. Direct the other hose into a container and gently turn on the water until the water runs clear, then reverse hoses and run again until clear. Reattach hoses to engine and refill reservoir.

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Q: I have replaced the thermostat heater control valve and flushed the cooling system Why is heater still blowing cold?
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