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GM recommends that a 50/50 mixture of ONLY GM Goodwrench DEX-COOL or Havoline DEX-COOL silicate-free coolant and clean drinkable water be used. Using other coolants can result in premature engine heater core or radiator corrosion. DEX-COOL extended life coolant, according to GM, should be good for 5 years or 150,000 miles whichever occurs first. If your car is a '98 and still has the original coolant, it's past time for a change. All this should be covered in Section 5, Service and Appearance Care, in your Owner's Manual and you may want to read what it has to say on adding/changing Coolant along with the cautions. Hot coolant is under pressure and can cause severe burns. So, if you haven't done this before and are not sure how, you may want to let someone that knows do it. Good luck.
I've gotten the low coolant messages before, but it was low.
Here's another for ya (just in case this happens to you and you're, like, WTF?): Coolant temp goes from 220 to 260 instantaneously, then back down to 220...then back to 260. Nope, not overheating. You could see air bubbles in the nozzles going into the coolant reservoir. I ran the car with the reservoir open and it resolved itself (oh, and topped off with coolant).