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Hey guys.
First and foremost I am not really a car person. I have an '04 Civic Si. I love to drive it. That's about all I know. I've read my manual but can't find anything about this. I also searched the forums.

Two months ago I brought my car out to school after a break. It is about a six hour drive. Car performed well, but at some point during the trip my A/C button no longer worked and when it was turned all the way to cool it was hot and turned all the way up it was simply stifling. We had to alternate between having it on cold (actually really hot!) and having it off. Trying to keep window fogging at bay without cooking ourselves. Also right before I took it to school I had a new head unit installed.

I got it to school, replaced a fuse in it and then I left it in the care of my cousin. I went to drive it one day and it wouldn't start. It was about 5 degrees, so we got a new battery for it (had the original in it).

When I bought the car over the summer the A/C definitely worked. To summarize now the A/C button will not light up if you depress it. With the dial turned all the way to cold it blows hot air, turned up extremely hot air.

My cousin and I didn't know if maybe this was one of those giant fuses? Could something have happened when they installed the head unit? Is this going to be thousands of dollars to fix? I'm going to bring it to the dealer over my spring break, but I was just looking for ideas. Thanks everyone!

 
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check fuse 14 under the steering wheel..