How often should I really change my fuel/water separator filter? This is one of the most crucial maintenance questions. There isn't a single answer that fits all applications, but the rule of thumb is to follow the OEM-recommended service intervals in your owner's manual, which are typically based on mileage or hours of operation. However, you should change it immediately if you notice: The indicator light on your Water-In-Fuel (WIF) sensor is illuminated. A decrease in engine power, hard starting, or rough idling. You are in an environment with a high risk of contaminated fuel. Remember, the primary job of the fuel/water separator is to remove water, which can cause catastrophic damage to your fuel injectors. When in doubt, drain the separator regularly and change it early. https://www.cq-longshine.com/cummins-filter-system-parts