Section 1. Short title

This Act may be cited as the Diesel Engine Flexibility Act or the DEF Act.

Sec. 2. Preservation of on-road vehicle and engine regulatory stability

Section 202 of the Clean Air Act (42 U.S.C. 7521) is amended by adding at the end the following:

(n) Preservation of motor vehicle and engine regulatory stability

(1) Guidance-based safe harbor

During the period of 10 years beginning on the date of enactment of this subsection, implementation of an inducement strategy or diesel exhaust fluid quality monitoring described in the covered guidance documents with respect to new and in-use motor vehicles and engines, including heavy-duty engines, heavy-duty vehicles, light-duty vehicles, light-duty trucks, and light-duty engines, shall not constitute a prohibited act under this title if the implementation is undertaken in a manner consistent with the covered guidance documents, to the extent such covered guidance documents apply to such a motor vehicle or engine that is required to comply with the 2007 standards or the 2010 standards, provided that such implementation is consistent with otherwise applicable certification requirements and useful life requirements under this title.

(2) Regulatory stability

(A) Temporary regulatory stability

During the period of 10 years described in paragraph (1), the Administrator may not promulgate, revise, authorize, or implement under this section or section 209(b) any regulation containing any standard or other requirement applicable to new or in-use motor vehicles or engines, including heavy-duty engines, heavy-duty vehicles, light-duty vehicles, light-duty trucks, and light-duty engines, that is more stringent than the 2007 standards and the 2010 standards, except that nothing in this paragraph shall prohibit the Administrator from—

(i)

approving software updates, repair strategies, diagnostic improvements, or monitoring approaches consistent with the covered guidance documents;

(ii)

enforcing prohibitions against defeat devices, defeat strategies, circumvention, fraud, tampering, or operations outside the conditions applicable to certified configurations under this title;

(iii)

implementing any recall, defect reporting, certification, warranty, or other administrative requirements that do not impose more stringent emission standards or materially increase compliance obligations applicable to such motor vehicles or engines that are required to comply with the 2007 standards or the 2010 standards; or

(iv)

approving a voluntary manufacturer action that maintains compliance with otherwise applicable certification requirements and useful life requirements promulgated under this title.

(B) Lead time

After the expiration of the period described in paragraph (1), any standard or other requirement promulgated, revised, or authorized under this section or section 209(b) applicable to new or in-use motor vehicles or engines, including heavy-duty engines, heavy-duty vehicles, light-duty vehicles, light-duty trucks, and light-duty engines, that revises or succeeds the 2007 standards or the 2010 standards shall—

(i)

apply for a period of no less than 3 model years; and

(ii)

take effect no earlier than the model year commencing 5 years after such new or revised standard or other requirement is promulgated, revised, or authorized.

(C) Required considerations

After the expiration of the period described in paragraph (1), in promulgating, revising, or authorizing a standard or other requirement under this section or section 209(b) applicable to new or in-use motor vehicles or engines, including heavy-duty engines, heavy-duty vehicles, light-duty vehicles, light-duty trucks, and light-duty engines, that revises or succeeds the 2007 standards or the 2010 standards, the Administrator shall consider—

(i)

the need to reduce unnecessary operational disruption, derates, shutdowns, and related operating restrictions for owners and operators of such motor vehicles and engines;

(ii)

the need to preserve operating conditions and engine configurations certified to comply with the 2007 standards or the 2010 standards that protect the durability of such motor vehicles and engines, selective catalytic reduction systems, diesel particulate filter systems, and related aftertreatment components;

(iii)

the effects of the proposed standard or other requirement on resale value, financing expectations, dealer inventories, and equipment availability across such motor vehicles and engines that are already in service or are available for sale;

(iv)

the costs of the proposed standard or other requirement to owners and operators of such motor vehicles and engines, including downtime, repair, service, logistics, and equipment replacement costs;

(v)

the need for nationally consistent requirements for such motor vehicles and engines; and

(vi)

the effects of the proposed standard or other requirement on harmonization across such motor vehicle and engine categories and engine families, equipment models, and shared engine or aftertreatment designs used across on-road categories.

(3) Rule of construction

Nothing in this subsection shall be construed to—

(A)

authorize permanent disablement of an emissions control system;

(B)

exempt any new or in-use motor vehicle or engine, including a heavy-duty engine, a heavy-duty vehicle, a light-duty vehicle, a light-duty truck, and a light-duty engine, from otherwise applicable emission standards promulgated under this title; or

(C)

prohibit the Administrator from approving software updates, repair strategies, diagnostic improvements, or monitoring approaches implemented consistent with otherwise applicable certification requirements and useful life requirements under this title.

(4) Definitions

In this subsection:

(A) 2007 standards

The term 2007 standards means the standards and associated certification and compliance requirements under section 86.007–11 of title 40, Code of Federal Regulations, as in effect on January 1, 2025.

(B) 2010 standards

The term 2010 standards means the standards and associated certification and compliance requirements under section 86.010–18 of title 40, Code of Federal Regulations, as in effect on January 1, 2025.

(C) Covered guidance documents

The term covered guidance documents means—

(i)

the guidance entitled Revised Guidance for Light Duty Vehicles, Heavy-Duty Diesel Engines and Nonroad Compression-Ignition (CI) Engines Using Selective Catalytic Reduction (SCR) Technologies, issued on August 11, 2025, and designated as IACD–2025–10; and

(ii)

the guidance entitled DEF Quality Monitoring Using Alternate Sensor Technologies, issued on March 26, 2026, and designated as IACD–2026–05.

Sec. 3. Preservation of nonroad vehicles and nonroad engines regulatory stability

Section 213 of the Clean Air Act (42 U.S.C. 7547) is amended by adding at the end the following:

(e) Nonroad vehicle and nonroad engine regulatory stability

(1) Guidance-based safe harbor

During the period of 10 years beginning on the date of enactment of this subsection, implementation of an inducement strategy, repair-related override, or diesel exhaust fluid quality monitoring described in the covered guidance documents with respect to a nonroad vehicle or nonroad engine, including a nonroad compression-ignition engine and equipment that uses such an engine, shall not constitute a prohibited act under this title if the implementation is undertaken in a manner consistent with the covered guidance documents, to the extent such covered guidance documents apply to such a nonroad vehicle or nonroad engine that is required to comply with the Tier 4 standards, provided that such implementation is consistent with otherwise applicable certification requirements and useful life requirements under this title.

(2) Regulatory stability

(A) Temporary regulatory stability

During the period of 10 years described in paragraph (1), the Administrator may not promulgate, revise, authorize, or implement under this section or section 209(e)(2) any regulation containing any standard or other requirement applicable to nonroad vehicles or nonroad engines, including nonroad compression-ignition engines and equipment containing such engines, that is more stringent than the Tier 4 standards, except that nothing in this paragraph shall prohibit the Administrator from—

(i)

approving software updates, repair strategies, diagnostic improvements, or monitoring approaches consistent with the covered guidance documents;

(ii)

enforcing prohibitions against defeat devices, defeat strategies, circumvention, fraud, tampering, or operations outside the conditions applicable to certified configurations under this title;

(iii)

implementing any recall, defect reporting, certification, warranty, or other administrative requirements that do not impose more stringent emission standards or materially increase compliance obligations applicable to such nonroad vehicles or nonroad engines that are required to comply with the Tier 4 standards; or

(iv)

approving a voluntary manufacturer action that maintains compliance with otherwise applicable certification requirements and useful life requirements promulgated under this title.

(B) Lead time

After the expiration of the period described in paragraph (1), any standard or other requirement promulgated, revised, or authorized under this section or section 209(e)(2) applicable to nonroad vehicles or nonroad engines, including nonroad compression-ignition engines and equipment containing such engines, that revises or succeeds the Tier 4 standards shall—

(i)

apply for a period of no less than 3 model years; and

(ii)

take effect no earlier than the model year commencing 5 years after such new or revised standard or other requirement is promulgated, revised, or authorized.

(C) Required considerations

After the expiration of the period described in paragraph (1), in promulgating, revising, or authorizing a standard or other requirement under this section or section 209(e)(2) applicable to nonroad vehicles or nonroad engines, including nonroad compression-ignition engines and equipment containing such engines, that revises or succeeds the Tier 4 standards, the Administrator shall consider—

(i)

the need to reduce unnecessary operational disruption, derates, shutdowns, and related operating restrictions for owners and operators of such nonroad vehicles or nonroad engines;

(ii)

the need to preserve operating conditions and engine configurations certified to comply with the Tier 4 standards that protect the durability of such nonroad vehicles and nonroad engines, selective catalytic reduction systems, diesel particulate filter systems, and related aftertreatment components;

(iii)

the effects of the proposed standard or other requirement on resale value, financing expectations, dealer inventories, and equipment availability across such nonroad vehicles or nonroad engines that are already in service or are available for sale;

(iv)

the costs of the proposed standard or other requirement to owners and operators of such nonroad vehicles or nonroad engines, including downtime, repair, service, logistics, and equipment replacement costs;

(v)

the need for nationally consistent requirements for such nonroad vehicles and nonroad engines; and

(vi)

the effects of the proposed standard or other requirement on harmonization across such nonroad vehicle and nonroad engine categories and engine families, equipment models, and shared engine or aftertreatment designs used across nonroad categories.

(3) Rule of construction

Nothing in this subsection shall be construed to—

(A)

authorize permanent disablement of an emissions control system;

(B)

exempt any nonroad vehicle or nonroad engine, including a nonroad compression-ignition engine and equipment containing such an engine, from otherwise applicable emission standards promulgated under this title; or

(C)

prohibit the Administrator from approving software updates, repair strategies, diagnostic improvements, or monitoring approaches implemented consistent with otherwise applicable certification requirements and useful life requirements under this title.

(4) Definitions

In this subsection:

(A) Covered guidance documents

The term covered guidance documents means—

(i)

the guidance entitled Revised Guidance for Light Duty Vehicles, Heavy-Duty Diesel Engines and Nonroad Compression-Ignition (CI) Engines Using Selective Catalytic Reduction (SCR) Technologies, issued on August 11, 2025, and designated as IACD–2025–10;

(ii)

the guidance entitled Clarification Regarding the Practice of Temporarily Disabling Aspects of an Emission Control System or Components to Repair or Maintain a Nonroad Engine, issued January 30, 2026, and designated IACD–2026–01; and

(iii)

the guidance entitled DEF Quality Monitoring Using Alternate Sensor Technologies, issued on March 26, 2026, and designated as IACD–2026–05.

(B) Tier 4 standards

The term Tier 4 standards means the standards and associated certification and compliance requirements under parts 1039 and 1068 of title 40, Code of Federal Regulations, as in effect on January 1, 2025.