New EPA Proposal Could Roll Back Some DEF-System Requirements For Diesel Engines

Months after telling manufacturers traditional diesel exhaust fluid (DEF) sensors are no longer mandatory for new trucks, tractors, buses, and other diesel equipment, the Environmental Protection Agency has turned its attention to engine deratement. If put into effect, engine deratement would become a thing of the past.

If you don’t know: Under current requirements, certain diesel engines derate when their emissions control system detects certain DEF-related problems. When that happens, the vehicle automatically limits its speed or engine power, just to be safe. But according to the EPA’s math, the cost of that safety is costing the trucking industry about $12 billion a year to comply.

Under their new proposal, the EPA would eliminate engine deratement entirely, scale back certain portions of emissions warranty requirements, delay implementation of some provisions from a 2023 heavy-duty emissions rule, and provide manufacturers with more flexibility as they work toward future nitrogen oxide (NOx) standards.

The way things are now, modern diesel engines rely on DEF to reduce harmful NOx emissions. (DEF comes in the form of a fluid injected into the exhaust stream.) Current regulations require onboard systems to monitor the DEF system and trigger increasingly severe warnings if it detects a malfunction. If the issue doesn’t get fixed, the truck will eventually enter a “derated” mode to encourage the driver to fix their emissions equipment.

But under the newly proposed rule, the EPA would get rid of deratement for newly manufactured highway diesel engines. If put into place, drivers would still get visible or audible warnings if a DEF system failure is detected… just without the speed or power decrease. (The agency is also looking for public feedback on whether the same should be done for existing diesel vehicles and equipment already in service, but that’d come later.)

The EPA estimates that doing away with deratement would save the trucking industry anywhere from $4,000 to $6,000 per diesel engine. But that would also drastically increase the amount of nitrogen oxide pollution in the air by 4.2% by 2030 and by 11.6% by 2055. The proposal is now subject to a 45-day public comment period and a public hearing before any changes could take effect. For now, existing DEF-system requirements and deratement rules remain in effect until the EPA completes the rulemaking process and adopts the changes.