Just Dispatched
A new federal proposal in the U.S. Senate aims to prevent trucks from being crippled by emissions controls during extreme cold. Introduced in November 2025 by Sen. Dan Sullivan (R-Alaska) and co-sponsored by Sen. Cynthia Lummis (R-Wyoming), S.3135 (unofficially the “Cold Weather Diesel Reliability Act”) would direct the EPA to allow manufacturers to suspend automatic engine derate in prolonged freezing conditions.
In practical terms, this bill seeks to carve out a cold-weather exception to current emissions rules, ensuring trucks can keep running at full power even if their diesel emission systems fault out in sub-zero temperatures. If enacted, the law would empower the EPA to waive or adjust Clean Air Act enforcement so that a diesel truck’s on-board diagnostic system does not forcibly slow down the vehicle when temperatures plunge for extended periods. By mandating a federal allowance for cold-weather operation, the bill could shield drivers and fleets from penalties or liability for temporarily bypassing emission-control inducements under defined winter conditions.
The Concern with DEF
There is mounting evidence that DEF-related derates in freezing weather have led to dangerous and costly incidents. For example, EPA Administrator Lee Zeldin noted hearing from motorcoach operators that an unexpected derate can leave a bus pulled over on the roadside, full of passengers, in unsafe conditions. “It’s a safety concern when a bus is forced to park unexpectedly on the side of the road…that’s all a product of poor government policy that didn’t think through the consequences,” Zeldin said of the prior DEF rules. In the trucking world, the economic toll is significant: One estimate by the Small Business Administration projected that sudden DEF-induced shutdowns were costing farmers who rely on diesel equipment, over $700 million annually in lost productivity.
The Industry Reacts
The trucking industry and other stakeholders have overwhelmingly welcomed efforts to prevent cold-weather engine shutdowns. The ATA praised the move, with ATA’s environmental affairs director noting that it “will avoid unnecessary towing costs and equipment downtime” while still preserving emissions benefits. ATA and state trucking groups had long argued that derating engines for a minor sensor fault was an overreach when it compromises safety. The Owner-Operator Independent Drivers Association (OOIDA) likewise celebrated the EPA’s guidance as “common sense”.
Truck and engine manufacturers, for their part, have shown support at the changes. The Truck & Engine Manufacturers Association (EMA) issued a statement “welcom[ing] the new guidance” and confirming that its members are preparing software updates as fast as possible. Engine makers like Cummins Inc. also publicly backed the push for more reasonable DEF policies.