I have a 2004 Nissan Quest Van. This belt was replaced about a year ago. Whenever I start the car, it squeaks like crazy. If I turn off the AC and turn it back on immediately, the squeaking goes right away. If I don't turn it off, it goes away in about a minute. I had asked a mechanic to spray belt dressing on it when they were doing some work, and they said they had never heard of doing that and didn't recommend it. Any thoughts on how to address this? I assumed if it just needed tightening it wouldn't quit so quickly when it was turned off and back on.

Its fine to use belt dressing. But have you inspected all your pullies and rollers on the belt path. Most newer cars don't have a place to adjust tension on a belt. They use a spring loaded tensioner. You can apply the belt dressing yourself and any auto store should have it. Also you need to check the condition of idler bearings you may have.

I'm not a fan of belt dressing--it only masks a more fundamental problem. There's no need for it on such a new belt, especially if it's an OEM belt. My bet is that the idler pulley or tensioner is squeaking. The idler pulley/tensioner should normally be replaced when the serpentine belt is changed--usually around 80,000 miles. Try spraying the inside of the tensioner with WD-40--it may quiet the spring mechanism. If the problem persists, replace the idler pulley/tensioner. Sometimes you can get lucky and just replace the idler pulley. Easy job.