Engine temp/coolant light problem when using heat

Recently the temp sensor on my car goes all the way to H and then the light comes on saying I need coolant. It only seems to happen when I have the heat and/or defrost on – if I turn the heat off the temp goes back to normal within a minute or so. (no problem using rear defrost)

First time it happened I pulled over, checked coolant level and added a little. The next time it happened I looked under the hood the next morning and coolant level was just fine.

Happens more often when I’m driving around town than on highway. Usually within about 5 minutes of driving in the morning the engine temp is all the way to H.

Any ideas welcome. Thank you!!

I drive a 2008 Ford Focus with about 50k miles.

You might want to have someone check and see if the radiator cooling fan is running when in the defrost mode. Most vehicles automatically run the air conditioning system when in the defrost mode. The compressor then forces refrigerant thru the condenser which is positioned in front of the radiator and this creates heat. This heat from the condenser is transfered into the radiator. If the radiator cooling fan isn’t running while in the defrost mode, and you’re driving at slow speeds, not enough air is passing thru the condenser/radiator to prevent the engine from overheating. But if you’re driving at fast speeds, enough air is passing thru the condenser/radiator where the radiator cooling fan isn’t required, and the engine doesn’t overheat.


Will do. Thanks so much!

Have you taken the actual radiator cap off and checked the coolant level in the radiator? Or are you only looking at the overflow reservoir? You should check the radiator. Only do it when the car is completely cold. If you find that the radiator is low you need to have a shop find your leak.