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EricFromCT, ASE Certified Technician
Category: Car
Satisfied Customers: 4036
Experience:  ASE Master certified L1, L2, Diesel Certified 16 years experience
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Honda Civic EX: 2003 Honda Civic EX Heater ProblemsI have

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2003 Honda Civic EX Heater Problems

I have been having a problem with my heat/air and defrost in my car. The problem is that it has been taking a long time for hot air to come out or cool air. And the defroster does not seem to be working at all. So I took it into a local shop. They said I needed a new thermostat and gasket and got me those and replaced them. Things never really seemed different at all and so a month or so later I took it back again and they said that it must have been a faulty thermostat and replaced it under warranty. Again I see no change after having driven the car. The hot/warm air seems to be working very intermittently and the defroster practically not at all. I suggested they look at my coolant and they said that it was fine. I don't want to go back a third time to the same repair shop. What could be wrong with my car if it is not the thermostat?"

Started in winter. I would say it started gradually because it was intermittent. This was the pattern: no defroster, no heater. But then, after 20 minutes - enough heat to make me feel like "maybe this car just takes awhile". And then - next time - nothing. And then - same. It gradually became worse.

A mechanic friend has told me that this may be consistent with a slow leak in the head gasket i.e. slow leak of coolant = intermittent heat/air. They said that if the proper compression test had been done as a diagnostic, it would have detected this.

Thank you, Michael
Hi. Your friend is right that lack of heat could indicate air is getting into the cooling system from a possible head gasket concern. It sounds like the thermostat is not the problem. Especially since it was replaced twice. The problem could be a plugged heater core or the problem could be in the heater box itself. Most common the air/mix motor goes bad. This is what mixes the hot and cold air in the heater box.
That diagram shows the air mix motor. What you can do to check, is to run the vehicle until the engine is warm. About 20 minutes or so. Then check the heater hoses that go into and out of the heater core at the firewall. If the hoses are both equally hot then I would suspect the heater core is not the problem. If one of the hoses is cooler than the other I would suspect a plugged heater core.
Please let me know if you need more info on this. Thanks, Eric
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