All coolants have a recommended interval. You should flush the radiator off the old coolant liquid before this interval and should replace it with a new coolant. The interval for replacing the coolant depends on the type and brand of it. But you should check the coolant liquid for signs of deterioration even before reaching this limit. Usually, they are a part of the scheduled maintenance. To check if your car cooling system needs new coolant, look for the signs of rust or leakage in the system. Also, you can check the coolant level by opening the coolant reservoir cap.