The Environmental Protection Agency on Tuesday said it would put forth a rulemaking in the Federal Register seeking information to help it to permanently end diesel derates.
The action follows an August 2025 move to change guidance to manufacturers and introduce a less harsh schedule of derates for diesel equipment, both on- and off-highway.
"As a follow up to new guidance in August 2025 reversing deratements that were harming farmers and truckers, EPA is pursuing rulemaking opportunities to provide further relief for Americans," according to a release from EPA. "To strengthen the agency’s ongoing efforts, EPA is demanding information from major diesel engine manufacturers on critical data from DEF system failures."
[Related: EPA announces end to 5-mph DEF derates for new and existing trucks]
"As part of that regulatory process to hopefully be able to PERMANENTLY eliminate the deratements ENTIRELY, I am sending a mandatory data collection request today to manufacturers regarding DEF system failures to quickly evaluate ongoing issues independently and launch the next step Americans are DEMANDING," EPA Administrator Lee Zeldin wrote on Twitter. "On behalf of our nation’s truckers, farmers, and others, WE ARE GOING TO GET THIS DONE at TRUMP SPEED!"
The data collected will allow EPA to independently evaluate ongoing system failures and help inform next steps the agency takes in 2026 "pertaining to DEF."
"Sudden speed losses and shutdowns caused by DEF system failures compromise safety and productivity," EPA said. "The Trump EPA’s guidance, which significantly reversed deratements, called on manufacturers to revise DEF system software in existing fleets to prevent these sudden shutdowns and give operators more time to repair faults."
Now EPA said it recognizes that "to improve systems, the product designs and materials for these systems must also be the highest quality possible."
The action on Tuesday sought to "address any potential insufficiencies with system parts to better advance future rulemaking and reduce system failures."
“EPA understands this is a massive issue, which is why we have already established commonsense guidance for manufacturers to update DEF systems,” said EPA Administrator Zeldin. “Today, we are furthering that work and demanding detailed data to hold manufacturers accountable for the continued system failures. The Trump EPA is committed to ending unnecessary frustrations and days lost on the road and in the field for American farmers and truckers.”
Under Section 208(a) of the Clean Air Act, EPA can require manufacturers to "provide information needed to assess whether emission control systems are functioning properly and whether manufacturers are meeting their obligations to identify and fix defects," EPA wrote.
As such, the agency is demanding info on the top 14 on- and off-highway manufacturers that combined account for more than 80% of all products used in DEF systems.
EPA specifically wants data on warranty claims, failure rates, and repair information for Model Years (MY) 2016, 2019 and 2023 emission control products. Overdrive readers ranked MY 2015-2019 as some of the least reliable trucks when it comes to aftertreatment systems in prior surveying and reporting on impacts of decades of emissions regulations on heavy-duty trucks.
EPA gave manufacturers 30 days to provide the requested information, warning that failure may result in additional inquiries or penalties. The agency said it's been meeting with manufacturers to track their progress on responding to its new, relaxed derate guidelines.
[Related: California 'smog check' rule 'discriminates' against out-of-state truckers: EPA]
Additionally, EPA said it was actively working on a proposal for the reconsideration of the 2022 Heavy-Duty Engine and Vehicle NOx rule.
"It is being thoroughly assessed whether derates may no longer be necessary for compliance," EPA wrote of the 2022 rule.
Also, EPA on Monday issued guidance to manufacturers saying that under the Clean Air Act, "Americans have the right to repair their farm and other non-road diesel equipment," including broken DEF systems.
EPA's move to end derates also follows the Department of Justice announcing it would no longer enforce criminal penalties for "diesel deletes," though many owner-ops remain wary of that.
