On May 18, 2026, the U.S. Environmental Protection Agency (EPA) published a proposed rule delaying Clean Air Act (CAA) “Tier 4” requirements for light-duty and medium-duty for two years. The Office of Advocacy (Advocacy) filed a comment letter on July 6 supporting the EPA’s proposal while urging the agency to review existing “Tier 4” requirements and eliminate diesel exhaust fluid (DEF) mandates.
The EPA’s action is part of two separate, but coordinated, rulemakings. The second rulemaking, which will follow the delay of the Tier 4 standards, will include a more thorough review of Tier 4 regulations. The EPA’s proposed delay will prevent small businesses from having to comply with standards that the agency is going to review and possibly change soon.
DEF mandates have created a significant regulatory burden for small businesses and are currently one of Advocacy’s “Most Wanted” regulatory reforms. To meet the EPA’s Tier 4 standards, small businesses have had to adopt selective catalytic reduction (SCR) systems which rely on DEF for nearly all their on-road diesel trucks as well as many types of non-road vehicles (such as tractors and construction equipment). Small businesses have told Advocacy that DEF causes engines to unexpectedly shut down and significantly reduces useful engine life.
COMMENT LETTER:
(PDF, 217 KB)
CONTACT: Nick Goldstein
EMAIL: nick.goldstein@sba.gov
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