What's the Biggest Sign You Should Change Your Engine Coolant?
The biggest and possibly most expensive sign you need to change your engine coolant is your car’s engine overheats. Overheating is usually a sign that something minor was left to get worse. Know the color of your coolant when it is new. The three main colors are light green/yellow, orange and black. Check your coolant for proper level and color at least once a week. When rust starts appearing in the coolant, you'll want to change the engine coolant soon. Rust, which shows up as a reddening of the coolant, clogs the cooling system, stifling coolant flow. This will eventually cause your engine to overheat.
For more info, see DIY Car Maintenance Servic: What to Do Yourself