Engine coolant loses some of its protective qualities after a period of time. This is why you should replace the engine coolant at 30,000 mile intervals if your engine has the traditional coolant (the green fluid) and 60,000 mile intervals for most extended life coolants (the red fluid). Some manufacturers will state 100,000 mile intervals for extended life coolant but in reality it should be replaced more often. When this service is performed, the entire cooling system is inspected to determine if hoses or other components need replacement. Coolant hoses are recommended to be replaced every 4 years, regardless of how many miles they have on them.