The Diesel Engine Flexibility Act aims to amend the Clean Air Act to ensure regulatory stability for motor vehicles, motor vehicle engines, nonroad vehicles, and nonroad engines. Its primary purpose is to prevent the immediate imposition of more stringent emission standards, thereby providing a period of predictability for manufacturers and operators. For a period of 10 years following enactment, the bill establishes a "safe harbor" for implementing specific inducement strategies or diesel exhaust fluid (DEF) quality monitoring practices, provided they are consistent with designated guidance documents. These practices, when applied to vehicles and engines complying with 2007, 2010, or Tier 4 standards, will not be considered prohibited acts under the Clean Air Act. During this 10-year stability period, the EPA Administrator is explicitly prohibited from promulgating, revising, or implementing any new emission standards that are more stringent than the 2007 and 2010 standards for on-road vehicles or the Tier 4 standards for nonroad engines. This prohibition includes heavy-duty and light-duty vehicles and engines, as well as nonroad compression-ignition engines. However, the Administrator is still permitted to approve software updates, enforce prohibitions against defeat devices, and implement administrative requirements like recalls, as long as these actions do not impose more stringent emission standards or materially increase compliance obligations. The bill also allows for the approval of voluntary manufacturer actions that maintain compliance with existing certification requirements. After the 10-year period expires, any new or revised emission standards must apply for a minimum of three model years and take effect no earlier than five years after their promulgation. When developing these future standards, the Administrator must consider several factors, including the need to reduce operational disruption, preserve engine durability, assess impacts on resale value and financing, evaluate costs to owners, ensure national consistency, and promote harmonization across vehicle and engine categories. Crucially, the bill clarifies that it does not authorize the permanent disablement of emission control systems, nor does it exempt any vehicle or engine from otherwise applicable emission standards. It also does not prevent the Administrator from approving software updates, repair strategies, or diagnostic improvements that are consistent with existing certification and useful life requirements.