What mileage is too high for a Duramax?

There is no single mileage cutoff for a Duramax. A 200,000-mile truck with records, clean cold starts, stable temperatures, a healthy transmission, and little rust can be a better buy than a neglected 120,000-mile truck. Use mileage as a pricing signal, then inspect the platform-specific risks: injector history on early LB7 trucks, cooling and head-gasket symptoms on older trucks, emissions-system history on later trucks, transmission service, front-end wear, rust, and signs of heavy towing. For a buyer shortlist, prefer documented maintenance, stock or well-supported modifications, clean title history, and a seller who can explain how the truck was used.