Diesel Reliability in Cold Climates Act of 2025

New bill to authorize manufacturers to suspend engine power-reduction functions and engine shutdowns in diesel vehicles during periods of cold weather.

A new bill is being reviewed by the Committee on Environment and Public Works, with the goal of allowing the Environmental Protection Agency (EPA) to authorize manufacturers to suspend engine power-reduction functions and engine shutdowns in diesel vehicles during periods of cold weather.

Named the Diesel Reliability in Cold Climates Act of 2025, the bill is sponsored and co-sponsored by Republican senators Dan Sullivan (Alaska) and Cynthia Lummis (Wyoming). It focuses on the Arctic temperatures at which diesel exhaust fluid (DEF) is known to freeze.

EPA Administrator Lee Zeldin previously urged engine and equipment manufacturers to review system software to prevent sudden power loss resulting from engine derating. However, the senators argue that automatic engine shutdowns and power reductions due to emission-control failures in extreme cold pose a potentially life-threatening risk.

Diesel Reliability in Cold Climates Act of 2025

In rural regions with prolonged freezing conditions, the storage, supply, and functionality of diesel exhaust fluid systems are often unreliable and logistically unfeasible year-round. Automatic engine shutdowns and power reductions caused by emission-control failures in extreme cold present serious risks that may endanger lives, the proposal states.

The Diesel Reliability in Cold Climates Act of 2025, according to the official release, does not exempt compliance with any emission standards under the Clean Air Act, emphasizing that protections against emissions were never intended to endanger human safety or hinder critical mobility.

The bill is intended to apply to both on-road and off-road diesel vehicles and equipment. It also establishes two main mandates for the EPA, each of which must be implemented within 180 days of the bill’s passage.

The EPA would be required to revise its regulations to allow manufacturers to suspend power-reduction or engine-shutdown functions. This temporary mode would be permitted when an emission-control failure is triggered at temperatures at or below zero degrees Celsius (0 °C/32 °F).

Notably, the function would only be authorized if the engine’s continuous maximum performance is necessary to prevent workplace hazards, equipment failures, or the loss of essential transportation capability in remote areas, according to the Congressional statement. Once temperatures rise above freezing, the engine would be required to return to normal emission-control operation.

Year-round exemption

Additionally, the Diesel Reliability in Cold Climates Act would grant a year-round exemption from DEF system requirements for specific vehicles. This exemption would apply to vehicles operating primarily north of 59 degrees north latitude—mainly in Alaska—or to those facing operating conditions where prolonged low temperatures make the use of DEF systems impractical.