Skip the auto shop - Our mechanics make house calls

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

asked by on

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.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: My car won't start

This may be related to many different possibilities, some of which may be related to a lack of spark and some of which may be related to a fuel delivery problem or a combination of both. For example, if the...

Q: Heat won't turn on with car running.

Hi there. With the Fusion vehicles, I recommend turning the defroster on high before you shut off the car to the day before. This way when you turn your car on by remote start, the defroster system will be working...

Q: 1999 ford expedition, replaced turn signal switch, now the rear hazards flash all the time

Hello there. This issue actually tends to happen quite a bit. It's usually caused by an electrical short either at the relay switch that triggers the hazards and individual right and left turn signal lights. You can resolve this issue...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...
P2428 OBD-II Trouble Code: Exhaust Gas Temperature Too High Bank 1
P2428 code definition A P2428 trouble code signifies that the PCM has detected a problem in the exhaust gas temperature sensor circuit in bank 1, which subsequently contains the number one...