Okay, so I realized that my heater stopped working. It blows cool air when on heat and it's winter time now in texas! My windows fog up a little bit but not enough to distract my driving. I was told it could be my heater core or possibly even my thermostat. It's definitely NOT the water or antifreeze level. That has been checked. My check engine light is on as well. Has been since my heater stopped working. My temperature level stays at half way. Always has since I had the car. Help me please! Anyone have a clue?? It's an '02 Kia Optima.

Check engine light should be a separate issue. Fog on windows could just be moisture and not a leak in the heater core and if anti-freeze it would have an odor and be smudgy to clean off glass. If you are running just water you are trashing your cooling system with corrosion - need to know that.

Not the thermostat if temp level is steady at norm.

Ok: Coolant is warm enough but doesn't transfer heat at the core. This is either because of a diverter door not directing air to flow thru the core or coolant can't pass thru. Look for the hoses that go to the core at firewall. Is there a device that looks like it regulates coolant flow to the core? If so that could be the problem. If not then feel the hoses - generally a slightly smaller one is inlet and larger for outlet back to engine. They should be warm. When heater works the outlet would be notably cooler. If neither are warm at all the flow just isn't getting there and if both are warm/hot I suspect thte diverter door controlled by you temp selector at control panel.

Do some observations. See if it can blow warm air on lowest fan setting and hit back for more ideas. It's likely that the core is plugged so a general look at the coolant for how clean it appears would be tel-tale,

T
Tom Greenleaf - MetroWest Boston - USA
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http://www.autoacsystems.com/tomgreenleaf/