DEF Act

The Diesel Engine Flexibility Act (DEF Act) amends the Clean Air Act to freeze certain motor vehicle and nonroad engine emission standards at 2007/2010 and Tier 4 levels for 10 years, provides a safe harbor for following specific EPA guidance on inducement strategies and diesel exhaust fluid monitoring, and sets lead-time and consideration requirements for any future stricter standards.

Sec. 2, Preservation of on-road vehicle and engine regulatory stability: For 10 years, following two specific EPA guidance documents (IACD-2025-10 and IACD-2026-05) on inducement strategies or diesel exhaust fluid quality monitoring for vehicles and engines meeting 2007 or 2010 standards is not a prohibited act, as long as it is consistent with certification and useful life requirements. During that 10-year period, the EPA Administrator cannot issue any regulation stricter than the 2007 and 2010 standards for new or in-use motor vehicles or engines, with exceptions for software updates, enforcement against defeat devices, recalls, and voluntary manufacturer actions. After the 10-year period, any new standard that revises or succeeds the 2007 or 2010 standards must apply for at least 3 model years and take effect no earlier than 5 years after promulgation, and the Administrator must consider operational disruption, durability, resale value, costs, national consistency, and harmonization.

Sec. 3, Preservation of nonroad vehicles and nonroad engines regulatory stability: For 10 years, following three specific EPA guidance documents (IACD-2025-10, IACD-2026-01, and IACD-2026-05) on inducement strategies, repair-related overrides, or diesel exhaust fluid quality monitoring for nonroad vehicles and engines meeting Tier 4 standards is not a prohibited act, as long as it is consistent with certification and useful life requirements. During that 10-year period, the EPA Administrator cannot issue any regulation stricter than the Tier 4 standards for nonroad vehicles or engines, with exceptions for software updates, enforcement against defeat devices, recalls, and voluntary manufacturer actions. After the 10-year period, any new standard that revises or succeeds the Tier 4 standards must apply for at least 3 model years and take effect no earlier than 5 years after promulgation, and the Administrator must consider operational disruption, durability, resale value, costs, national consistency, and harmonization.