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My air vents blow out air. That's it. They do it well, too. Two weeks ago, when I turned it to high heat, the air was HOT.

But now it's just air.

The AC doesn't work either, but I know where that problem comes from. But what would cause the heat to cut out? I thought the hot air was just fanned in from the engine, so it should be heated automatically...this seems strange to me.

EDIT: I drive a 97 Subaru Legacy; I know nothing else. :)

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Is your radiator low? A low coolant level could reduce the amount of hot coolant that enters the heater core. You might want to consider listing what you've checked so far, what kind of vehicle you have, etc., and if you've checked nothing, list that too. :) –  jmort253 Apr 5 '12 at 7:24
    
Interesting question - my car heater has started to get cooler. On a good run I get full temperature, but if I am parked with the engine running and the heater on full, it cools right down to near ambient temperature. –  Rory Alsop Apr 5 '12 at 10:38
    
@Rory That's the opposite of what mine would do. While I was moving the coolant temperature, and therefore heat output, would drop (due to all the airflow that was cooling my overactive thermostat), but when parked I'd get heat since the radiator couldn't mistakenly dissipate too much heat... :-) –  Brian Knoblauch Apr 5 '12 at 17:37
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up vote 7 down vote accepted

Check the coolant temperature if you can (if it's an OBD-II car, some readers can give you coolant temperature). If you've got a coolant temp gauge, see if it looks any lower at all (most are pretty hard to read though, one tiny tick down can be a HUGE difference in coolant temperature).

I suspect a failed thermostat that's sticking open. I had that recently on one of my cars. Temperature is supposed to be around 190F. Car made tons of heat. Temp started only getting up to 160-180F and the heater basically quit working (amazing since it's not really much difference in coolant temp!). New thermostat installed, temp is back to 190F and the heater works great again!

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Brian is right - this is almost certainly the cause. Remember that your cabin heater is using waste heat from the engine coolant system. If that system is running cold, there won't be a lot of waste heat to use. –  Bob Cross Apr 5 '12 at 11:58
    
I don't imagine thermostats are particularly expensive components? –  Aerovistae Apr 5 '12 at 18:04
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Probably not. My replacement costs were $12 for the thermostat, $26 worth of coolant, and $85 (an hour labor) to have a local shop do it so I wouldn't be stuck cleaning up the big mess that happens when the upper radiator hose is removed. :-D –  Brian Knoblauch Apr 5 '12 at 18:09
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