Normally, states aren't allowed to set their own pollution rules for vehicles—that's the federal government's job—but California can ask for special permission (called a waiver) to enact stricter standards than the rest of the country, since it has historically had serious air pollution problems. The EPA had granted California such permission to enforce tougher smog-forming pollution limits (specifically targeting nitrogen oxides, or NOx) on new heavy-duty trucks, buses, and off-road diesel equipment starting with 2024 models. This resolution would cancel that permission, meaning California would not be allowed to enforce those stricter emissions rules on truck and engine manufacturers, which would instead continue following the less strict federal standards.