Knock sensor and liftgate on 2007 Chrysler Town & Country

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Have an '07 with 94K miles on it. We just bought it two days ago and the check engine light came on this morning. Had the guys at AutoZone hook it up to their diagnostic computer and it came back with an error code for the knock sensor. Could someone give me an idea of what it costs to replace this?

I also had some trouble with the back liftgate this morning. When I hit the button to open it, it would only go about a foot, jam, then come back down. Finally had to lift it open manually (didn't take any unusual force to do so). Now it lifts completely using the key fob, but makes a popping sound on the left hand side. Any ideas?
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