Q: Overheats when heater is on, heater blows cold air.

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I just had the heater core bypass hose assembly replaced two weeks ago due to a very bad leak. Car was running fine after the repair, no temp problems, until I turned the heater on today. Within five minutes the temp gauge was almost all the way up while the heater was still blowing cold. The heater used to work great before the repair, save for a slight coolant smell when turned on. Now it blows cold while the motor overheats! That's the opposite of what I need!

My car has 233000 miles.
My car has an automatic transmission.

Since you had cooling system work done recently, one possibility is the cooling system is air bound. If air is trapped within the cooling system and not properly bled from the system after a repair, the air can cause the coolant to not circulate properly. This can lead to overheating and no hot air from the heater. Another cause that can simply be coincidence is a stuck thermostat. The function of the thermostat is to allow the engine to reach a certain temperature before opening and allowing coolant to flow throughout the cooling system. If the thermostat is stuck closed and fails to open, the coolant will not flow properly and cause the engine to overheat. The heater will blow cold air because hot coolant is not flowing to the heater core behind the dashboard to provide heat for the interior. Have a certified technician diagnose the engine overheating as soon as possible. Continued driving while overheating can damage the engine.

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