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Thermostat is not allowing the engine to warm up enough to produce heated coolant to the heater core. Heater control valve is faulty or not opening. Low coolant level; coolant level is insufficient to pump into the heater core. Blocked or damaged heater core. Start with the thermostat, that's very common. While you're at it, make sure the coolant level is up. After that, if you suspect a blocked heater core, try backflushing the heater core. Turn the heat on full to open the valve, then with the heater hoses removed, run water BACKWARDS through the heater core. As you remove the heater hoses. Don't use much water pressure, if you go over 15 PSI pressure you could blow the heater core and then you have real problems. Typical water pressure at a house is 50 to 120 PSI, so if you use your water hose make sure you control the pressure.

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18y ago

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