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Q: Heater blows cold.

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After using coolant leaking stopper 3 months ago, heater has blown cold weather. I changed thermostat, Flushed heater core radiator 2 times, and replace the coolant reservoir cap, but the issue never been resolved. The car does not get overheat and heating gage raised to the middle and stays cosistant untill turning off the car and getting cool down. When turning heat on there is hot blow for seconds and eventually dropt to cold one.

My car has 70000 miles.
My car has an automatic transmission.

Hi there. Leak stopper should only be used in emergency situations as it usually causes more problems than it "fixes". It sounds like the leak stopper may have settled in your heater core and is preventing the proper amount of coolant from passing through to provide heat inside the car. The only way to repair this is usually to replace the heater core and check the hoses for blockages and restrictions as well. There could also be a servo malfunction that has coincidentally surfaced. A visual inspection of the servo operation can usually identify a mechanical fault, but not an electrical fault. If you can gain visual access to the servo, turn the temperature to max cold then max heat and observe the movement. If the servo is moving both directions fully, then it is working and the fault is in the heater core. I suggest having a qualified technician perform an inspection to avoid replacing unnecessary parts. Your Mechanic has several technicians available that can assist you with a car has not heat inspection.

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