I am writing this based on my 1996 Cavalier 2.4L, but I think the same (or similar) coolant sensor system is used on all Cavaliers and Sunfires of that era, and perhaps on many other vehicles or different generations.

You may constantly or frequently get a "low coolant" warning light on your dashboard cluster but when you check your coolant reservoir tank, the coolant level is always fine. In that case it could be (1) the coolant level sensor switch, which is located on the outside of coolant reservoir tank. But more likely, the bobber on the inside of your coolant reservoir tank is stuck in its track at a low position inside the tank. That bobber has a magnet which is detected by the sensor. If the bobber gets stuck in a low position, then the sensor reports that the coolant level is low.

The coolant level sensor is located on the outside of the tank. It is easy to remove with a needle nose pliers and cheap ($11-$14 on Amazon as of January 2015) but I think that in most cases the sensor is NOT the problem. When I told my mechanic about the low coolant warning light, he immediately told me that I needed to replace the coolant reservoir-- and he told me that I could easily do so myself. He didn't tell me to replace the sensor. He knows his stuff. But I still thought I'd try replacing the sensor. I replaced the sensor but it didn't help. He was right. I don't think that there's much that can go wrong with a sensor that simply detects a magnet, but you can gamble on replacing it (and maybe return it to the store if it doesn't fix the problem).

If the problem is a stuck-bobber, then you can try to fix it in one of 3 ways:
1) Try tapping the reservoir (near the sensor) with a hammer and/or screwdriver butt firmly to jar loose the bobber-- but not so hard that you crack the reservoir tank. This worked for me one time but then it got stuck again about a year later and I couldn't jar it loose.
2) Empty and remove the reservoir tank. Then hold it upside down and tap it against a hard surface in order to jar the bobber loose. If the bobber can move freely, you'll hear it knock back and forth when you shake the reservoir tank. If the bobber is stuck, you won't hear it knock back and forth. If you try this method, I'd also recommend that you flush it with clean water several times (perhaps blasting it with a garden hose also) and shaking it back and forth so that the bobber is good and loose, and that any sediment and other problem-causing gunk is removed. Then replace the tank.
3) Replace the coolant reservoir tank. They are about $45 on Amazon as of January, 2015. They should come with the sensor built-in.

Removing the coolant reservoir tank is pretty easy and should only take about 20 minutes.
1) Drain the coolant reservoir with a turkey baster.
2) Use a pliers to pinch and slide-away the hose clamps.
3) Detach the 3 hoses from the tank. (They may be stuck on pretty hard so firmly twist and pull at the end near the tank, being careful not to break the hose.)
4) Secure the 3 hose so that they're pointed up and held above the tank level-- otherwise they might leak coolant. You could also clamp them shut.
5) Remove the sensor's electrical connector.
6) Unbolt the reservoir. If I remember correctly, it's held in by one 10mm hex-head screw at the front. The back is held by a tab which is pretty strong so you don't really have to worry about breaking it.