Why does the heat on my 2002 Chevy Malibu only run on 'high'?

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The heat only runs on high- no low or mid level. The heat comes out warm. The Ac works. Please help!

Thanks
I swithced the relay from the "high" to low and tried to run it, and the "low" heat still didn't work, and on mine, they're silver, not black, maybe that has something to do with it. This is in response to the last answer rcvd. Thanks