H.R. 9618: Diesel Engine Flexibility Act

This bill would change how the Environmental Protection Agency (EPA) can regulate diesel emissions controls for certain vehicles and engines under the Clean Air Act.

What it does for on-road vehicles and engines

For a 10-year period after the bill becomes law, the bill would create a “safe harbor” for certain diesel exhaust fluid (DEF) monitoring and inducement strategies if they are used in a way that follows specific EPA guidance documents. In plain terms, manufacturers and operators would be protected from violating the law when using those approved approaches on covered vehicles and engines, as long as they also meet existing certification and useful-life rules.

During that same 10-year period, the EPA would be barred from issuing new, more stringent rules for covered motor vehicles and engines that go beyond the 2007 and 2010 emissions standards. The bill applies this to both new and in-use vehicles and engines, including heavy-duty and light-duty categories.

The bill still allows the EPA to:

• approve software updates, repair strategies, diagnostic improvements, and monitoring methods that match the covered guidance;
• enforce bans on defeat devices, tampering, fraud, or bypassing certified configurations;
• handle recalls, defect reporting, certification, warranty, and similar administrative matters, so long as they do not create tighter emissions limits or materially increase compliance burdens;
• approve voluntary manufacturer actions that keep vehicles or engines in compliance.

After the 10-year period ends, any new or revised federal rule replacing the 2007 or 2010 standards would have to:

• stay in effect for at least 3 model years; and
• not take effect until at least 5 years after it is issued.

When setting future rules, the EPA would also have to consider possible disruptions to vehicle or engine operation, effects on resale value and financing, impacts on dealer inventories and equipment availability, compliance costs, downtime, repair and replacement costs, the need for nationwide consistency, and how rules affect harmonization across different vehicle and engine types.

What it does for nonroad vehicles and engines

The bill makes similar changes for nonroad diesel equipment and engines, such as construction or farm equipment using nonroad compression-ignition engines.

For 10 years, the bill would protect certain DEF monitoring, inducement strategies, and repair-related override actions from being treated as violations if they follow covered EPA guidance and comply with existing certification and useful-life requirements for equipment subject to the Tier 4 standards.

During that 10-year period, the EPA could not issue new, more stringent regulations for covered nonroad vehicles or engines beyond the Tier 4 standards. The same kinds of exceptions would remain available for software updates, repair strategies, diagnostics, monitoring, enforcement against tampering, recalls, and voluntary compliance actions.

After the 10-year period, any new or revised rule replacing the Tier 4 standards would also need to:

• apply for at least 3 model years; and
• take effect no earlier than 5 years after issuance.

The EPA would also have to weigh operational disruption, durability of emissions systems, resale and financing effects, equipment availability, compliance costs, nationwide consistency, and harmonization across equipment categories.

Limits and definitions

The bill says it does not allow permanent disabling of emissions control systems, does not exempt vehicles or engines from existing emissions standards, and does not prevent the EPA from approving certain software or repair measures that remain consistent with current certification rules.

It defines the “covered guidance documents” by referencing specific EPA guidance issued in 2025 and 2026, and it defines the “2007 standards,” “2010 standards,” and “Tier 4 standards” by reference to existing federal regulations as they stood on January 1, 2025.

Relevant Companies

• DAN — Dana Inc. could be affected because it supplies drivetrain and related components used in commercial and off-road equipment that may be subject to diesel emissions compliance changes.
• CMI — Cummins Inc. could be affected because it makes diesel engines and emissions-related systems used in on-road and off-road applications.
• DE — Deere & Company could be affected because its agricultural and construction equipment uses nonroad diesel engines subject to Tier 4-related rules.
• CAT — Caterpillar Inc. could be affected because it produces nonroad diesel engines and equipment covered by nonroad emissions standards.
• PCAR — PACCAR Inc. could be affected because it manufactures heavy-duty trucks and engines that are subject to on-road diesel emissions requirements.
• TTM — Traton SE could be affected through its truck brands if U.S. emissions compliance timelines and requirements change.
• OTIS — None found