Does 100% fuel-filter life prove a used Duramax has a new filter?

No. It shows the state of the service-life estimate, not proof that someone installed a new filter. The 2020 6.6L Duramax supplement explains that the fuel-filter life display is reset after replacement and returns to 100%. It also tells owners not to reset it at other times. That means the number alone cannot establish the service date or the part used. Ask for a receipt showing the truck, date, mileage, and filter part number. If the owner did the job at home, a parts receipt and maintenance log are still more useful than a dashboard photo. Missing paperwork does not prove dishonesty; it means the service remains unverified. When buying, put an undocumented filter service into your initial maintenance plan. Follow the schedule and procedure for that specific year. Do not treat a high remaining-life percentage as permission to ignore a water-in-fuel warning, a leak, or a drivability problem. Once the correct filter has been replaced, record the work and reset the appropriate indicator. Keep oil-life and fuel-filter reminders separate. A clean service history starts with one clearly documented job, and that is far more valuable to the next buyer than a screen full of perfect percentages.