A car's coolant light.

The coolant light glowing blue when you start your car is a warning that the engine isn’t yet up to its optimal running temperature—so don’t push the engine hard. Doing that when the engine is cold can reduce the longevity of the car's engine.

If a yellow coolant light pops on in the winter, it’s probably because your coolant level is low. "Coolant contracts in colder temperatures, so even if the car’s coolant level has fallen only a bit below normal, it could still trigger the system’s sensor," says John Ibbotson, CR's chief mechanic. Add coolant to the reservoir until the fluid reaches the "max cold" or "full cold" line. Do this when the engine is cold (turned off for at least 3 hours).

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A red coolant light temperature warning in the winter—more typically seen in warmer months—can be critical, because it means the car is overheating. "This is a serious problem, and the engine should be shut off immediately," Ibbotson says. "Have the car towed to a mechanic." Also, never unscrew the cooling system’s fill cap when the engine is hot; hot coolant can erupt and cause serious burns.

Check CR’s Car Repair Encyclopedia for our guide to your car’s dashboard warning lights plus many more auto-care tips.

Editor’s Note: This article also appeared in the November 2019 issue of Consumer Reports magazine.