Diesel Emissions Reduction Act of 2019 This bill reauthorizes through FY2024 and revises the diesel emissions reduction program. The program authorizes the Environmental Protection Agency (EPA) to provide grants, rebates, or loans for replacing diesel engines or retrofitting the engines with pollution control technologies. The bill modifies requirements governing how the EPA must prioritize projects when providing grants, rebates, and loans. Specifically, the EPA must recognize differences in how vehicles, engines, equipment, and fleets are used across the country as it prioritizes projects that include certified engine configurations, verified technologies, or emerging technologies that have a long expected useful life. Under current law, the EPA must use a certain percentage of the funds made available for the program to support state administered programs for reducing diesel emissions. If a state does not quality for funds, then the funds for the unqualified state must be reallocated to qualified states. This bill requires the funds for the unqualified state to be reallocated to the national diesel emissions reduction program.