New EPA Proposal Could End DEF Engine Derate Rule
WASHINGTON, DC – On July 14, the U.S. Environmental Protection Agency (EPA) proposed a major change to how diesel vehicles and engines respond when their selective catalytic reduction (SCR) systems detect diesel exhaust fluid (DEF) problems. Instead of requiring engine power reductions and vehicle speed limits, commonly called derates, newly manufactured diesel vehicles and engines could rely on visible or audible warnings if the rule is finalized. For owners of future diesel motorhomes and certain diesel tow vehicles covered by applicable light, medium, or heavy-duty regulations, the EPA proposal could reduce the risk of a DEF-related fault. Currently, as result of EPA regulations, these DEF faults unexpectedly limit speed or power, and still require drivers to maintain the emissions system — as required by the current derate rule.
Proposal Would Replace Derates With Warnings
According to the EPA’s July 14 proposal, the agency wants to replace required SCR-related engine derates and vehicle speed reductions with dashboard warnings or audible notifications. However, these replacements would apply only for newly manufactured diesel-fueled highway vehicles and engines, including light and medium-duty vehicles and heavy-duty engines. The proposal also covers new nonroad diesel engines and equipment.
SCR systems inject DEF into the exhaust stream to reduce nitrogen-oxide emissions. Current inducement strategies can restrict vehicle performance when the system detects insufficient. It applies when they detect poor-quality DEF, certain component failures, or potential tampering. The EPA says continued complaints from diesel operators prompted it to reconsider whether derates are the least restrictive way to encourage proper DEF use and emissions-system maintenance.
Motorhome Owners Could See Fewer Disruptions
The RV Industry Association says the proposal could be particularly relevant to manufacturers and owners of diesel-powered motorhomes equipped with SCR systems. RVIA says possible benefits include fewer unexpected derates caused by sensor or component failures, fewer service disruptions and lower warranty or diagnostic costs if false inducements decline. RVIA says the proposal also emphasizes better diagnostic accuracy while maintaining emissions compliance.
The proposal would not eliminate DEF, SCR systems, or the owner’s responsibility to maintain them. Drivers would still need to keep adequate DEF in the vehicle. Additionally, they would need respond to system warnings and avoid emissions-system tampering.
Existing Motorhomes Are a Separate Question
The EPA says the proposed rule applies to newly manufactured engines and vehicles. The agency is also considering separate guidance that could allow manufacturers to modify some vehicles already in use. Modifications would ensure their inducement strategies align with whatever changes are eventually adopted for new vehicles. That possibility is not yet a requirement or a guaranteed software update for existing motorhomes or tow vehicles.
Public Comments Remain Open
When the EPA published the new proposal to the DEF engine derate rule on July 14, it opened a 45-day public comment period that runs through August 29. Public hearings were held July 29 and 30.
Until a final rule is issued, diesel motorhome and tow-vehicle owners should not expect the proposal itself to change how their current vehicles behave. The immediate significance is what it could mean for future diesel motorhomes and qualifying tow vehicles — and whether EPA later creates a path for manufacturers to update compatible vehicles already on the road.
Gabriel Rivero
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