Owners of diesel-fueled vehicles including motorhomes and pickups have long complained about conditions that happen when diesel exhaust fluid (DEF) runs low. Under current guidelines from the U.S. Environmental Protection Agency (EPA), when DEF levels get low, the engine’s power output derates and performance drops significantly. However, the EPA is now changing that position and lifting diesel restrictions for vehicles.
On August 12, in Des Moines, Iowa, EPA Administrator Lee Zeldin announced the change in an official statement. Zeldin gave clear guidance urging engine and equipment manufacturers to revise DEF system software in existing vehicles and equipment to prevent these sudden shutdowns. The alteration would give operators more time to repair faults without impacting productivity or safety.
Why the EPA Is Lifting Diesel Restrictions Now
Many owners of diesel-powered motorhomes have complained about the derating when availability of DEF is limited but the destination isn’t and the diesel engines in the motorhomes suddenly reduce power significantly making it difficult to travel anywhere near highway speeds.
When DEF runs out or a system sensor fails, current systems can force a vehicle to drastically reduce speed or become inoperable. In many cases, vehicles are limited to as little as five miles per hour within hours of a DEF-related fault, causing significant disruptions in logistics, agriculture, and construction. Although this derate strategy was intended to ensure compliance with EPA’s Tier 4 Emissions Standards, it has caused needless frustration, operational delays, and real economic hardship for countless farmers, truckers, and equipment operators.
Many owners of large diesel-powered RVs have complained about this system as a number of failures have occurred with sensors so, even though the driver still has sufficient DEF, the failed sensor caused the RV to derate and operate in limp mode. Since the sensor issues were not uncommon, availability of replacement parts, especially in some rural areas where RVers like to travel, was problematic.
Zeldin stated that the complaints did not go unnoticed,“We have heard loud and clear from small businesses across the United States that the current DEF system is unacceptable. It is unacceptable that farmers, truckers, construction workers, and many other small businesses continually experience failures of diesel-powered equipment when they need it most — costing millions of dollars in lost productivity.” Zeldin continued, “Today, we are responding to those concerns by calling on manufacturers to take action to update their software and eliminate the unnecessary sudden loss of power and frustrating shutdowns that too many Americans have experienced.”
When the Implementation Will Take Place
Starting with model year 2027, EPA requirements state that all new diesel on-road trucks must be engineered to avoid sudden and severe power loss after running out of DEF. To fix the problem for vehicles already in use, EPA’s new guidance, developed in collaboration with manufacturers, will work to ensure that the necessary software changes can be made on the existing fleet.
In addition to providing certainty to manufacturers about how EPA wants this issue resolved, the agency is not requiring separate approvals beyond that provided in EPA’s guidance. In a statement released by the EPA, the organization wrote that its statement should serve as notification to the manufacturers to proceed with the fix and changes to 2027 model year vehicles.
What It Means for RVers
The changes mean RVers will have more time to get to a store, truck stop, or service center if they run low on DEF. For RVers traveling in rural areas where DEF can be hard to find, this will make a big difference. Instead of a sudden slowdown or speed cap, the updated rules build in a much bigger cushion.
Low DEF Grace Periods – New EPA guideline
| Vehicle Type | First Power Reduction | Final Speed Cap |
| Heavy-Duty Pickups & RV Tow Vehicles | 45 mph at 80 hours / 4,200 miles | |
| Motorhomes | Follow chassis guidelines (heavy-duty pickup or medium-duty truck) | Same as chassis type |
| Semi Truck Tractors | 15% power drop at 10 hours / 650 miles | 25 mph at 160 hours / 8,400 miles |
Since 2010, most on-road diesel trucks and many types of nonroad equipment (such as tractors and construction machinery) have used selective catalytic reduction (SCR) systems that inject DEF into exhaust streams to reduce nitrogen oxide (NOx) emissions.
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