Agencies Order Truck Manufacturers to Halt Forced Emergency Mode for Inadequate DEF Levels
The United States Environmental Protection Agency (EPA) has announced a new plan to address the issue of sudden power losses in diesel trucks caused by low Diesel Exhaust Fluid (DEF) levels. The EPA's plan, starting with model year 2027, requires all new diesel on-road trucks to be engineered to avoid sudden and severe power loss due to running out of Diesel Exhaust Fluid (DEF).
Current systems reduce engine power severely, sometimes limiting speed to about five mph, when DEF runs low or a DEF system sensor fails. These derates were intended to ensure NOx emissions compliance under Tier 4 standards but have resulted in significant economic hardship.
The EPA’s new guidance relaxes the strict 'inducement' requirements linked to DEF system issues, requiring manufacturers to design systems and software that avoid abrupt power loss while still protecting emissions integrity. For vehicles already in service, the EPA is working with manufacturers to deliver software updates without onerous regulatory approval procedures.
From 2027 onward, trucks will no longer be required to have software that causes severe power reductions related to DEF levels, effectively rolling back this regulatory requirement while maintaining emissions protections through alternative means.
This approach balances continued emissions control with more reliable engine operation, reducing the risk of unexpected shutdowns or slowdowns that disrupt users’ work. It provides certainty and flexibility for manufacturers to implement practical, durable solutions supporting both emissions goals and operational dependability.
The EPA's plan to eliminate sudden power loss is a sweeping move, developed in collaboration with manufacturers. The agency is also issuing new guidance for software changes to address DEF issues in vehicles already in use, applying to both used and new trucks.
The EPA's latest decision aims to champion everyday drivers and equipment operators by addressing their concerns about DEF and its impact on performance and reliability. Hundreds of thousands of drivers have removed their trucks' emissions equipment to avoid using DEF, citing performance and reliability concerns.
The top speed of heavy-duty semis won't be limited for much longer; only after about four work weeks will they be limited to 25 mph when running with low DEF levels. Engine derating will still be part of the strategy, but to a far lesser degree, with heavy-duty semis able to run for a full day with low DEF levels before performance is slightly restricted by 15%.
The EPA's announcement was made by Administrator Lee Zeldin at the Iowa State Fair. This is big news that will surely prove to be polarizing in due time. The EPA, under the Trump Administration, is conducting an about-face from its predecessors on environmental matters, as less than two weeks ago, the EPA announced its intentions to stop regulating greenhouse gas emissions.
Our platform has reached out to Ford, General Motors, and Ram for comment, but has not yet received a response. Major fines and penalties have been dished out to diesel aftermarket companies due to modifications that avoid using DEF.
It's important to note that DEF is a solution made from urea and deionized water, injected into the exhaust stream to reduce nitrogen oxide (NOx) emissions by as much as 99%.
This story now includes more details about the EPA's plan to eliminate sudden power loss and make the engine derating process more gradual.
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