02 Envoy automatic a/c

This is an '02 GMC Envoy. The climate control readout says 50 degrees F most of the time even though it is 90+ outside. Sometimes it drops as low
as -8 degrees. It's all over the place. Even when it displays 50 degrees, though, it still blows cold. Anything below 50 and it blows hot.
Today it started blowing hot, and there was a clicking from under the dash.
Is this just an external temp sensor gone bad? Aren't they about 20 bucks?
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