Today’s diesel trucks are marvels of engineering. You can waltz down to your local Ford dealer and drive away with a truck that produces 500 horsepower, 1,200 lb-ft of torque, and can pull up to 40,000 pounds. This modern power comes with some modern responsibilities. Today’s trucks use a bevy of emissions systems to clean up their exhaust, and one of them is Diesel Exhaust Fluid. When you run out of DEF or the sensor fails, your truck may derate or go into a full limp mode. In a reversal from past regulations, the United States Environmental Agency is considering getting rid of the derating requirement entirely.
The current administration has been reversing the federal government’s stances on many issues, including how the U.S. Environmental Protection Agency (EPA) and the Department of Justice (DOJ) enforce the Clean Air Act. In the past, the federal government pursued shops that performed emissions equipment deletes on diesel trucks. Last month, we reported how the Justice Department has decided that it will no longer pursue criminal charges under the Clean Air Act for allegedly tampering with emissions devices.
EPA Administrator Lee Zeldin has spent part of his tenure making deregulation a focus of the department. One area of EPA deregulation has been easing rules for what should happen when a truck either runs out of Diesel Exhaust Fluid or the DEF equipment otherwise fails. EPA Administrator Lee Zeldin had previously ordered truck makers to stop forcing trucks into immediate limp modes due to low DEF. Now, the EPA is considering eliminating the requirement for limp mode entirely. It also wants manufacturers to report on their experiences with equipment failures. It’s a move that could make a large impact.
What Is DEF?
If you own a modern diesel car, SUV, or truck, you’re already well acquainted with DEF, and I wouldn’t be surprised if you have a jug of the stuff in your trunk or bed right now. But why do modern diesels use DEF, and why is derating these diesels when they run out a big deal?
According to Diesel Power magazine, the implementation of diesel emissions equipment was gradual. Back in the 2000s, buyers of trucks found an Exhaust Gas Recirculation (EGR) system in their engine bays. EGR systems reduce emissions by recirculating a portion of an engine’s exhaust back into the intake. Even your modern gasoline-fueled car has an EGR system. Next came the Diesel Particulate Filter (DPF), which is an exhaust aftertreatment system designed to trap particulate matter before it leaves the vehicle.
As emissions requirements demanded diesels to run even cleaner, emissions equipment evolved. The Selective Catalytic Reduction (SCR) was an important advancement in diesel emissions reduction technology. 2010 model year heavy diesels have SCR to meet the EPA’s strict regulations. Any light-duty diesel that didn’t already use SCR phased in its use during the early 2010s.
Selective Catalytic Reduction utilizes a catalyst in conjunction with Diesel Exhaust Fluid to clean the exhaust stream of a diesel vehicle. From Diesel Power magazine:
Selective catalytic reduction technology uses ammonia to break down NOx emissions produced during diesel combustion into nitrogen and water. SCR has become the technology of choice for a majority of truck and engine manufacturers to meet the stringent 2010 emissions standards set by the EPA for heavy-duty trucks. The biggest benefit of SCR for the vehicle owner is in the fuel savings the technology provides. Because SCR deals with NOx outside the engine, manufacturers are once again able to tune their engines to run more efficiently and produce more power. The increase in engine efficiency also leads to a reduction in particulate matter, resulting in less frequent regeneration of the DPF and adding to the increased fuel economy.
SCR works by first routing exhaust gases through an oxidation catalyst, which removes hydrocarbons and converts a small amount of NOx to NO2. The next step requires an injection of an aqueous urea solution, DEF, into the exhaust stream at a precise dosing rate. Exhaust fluid is converted into ammonia, which reacts with the remaining NOx in the SCR catalyst to produce harmless nitrogen and water. A final catalyst is sometimes installed downstream of the SCR catalyst, which is designed to remove any remaining ammonia from the vehicle’s exhaust.
Diesel exhaust fluid is a mixture of synthetic, high-purity, automotive-grade urea and deionized water. This liquid is clear, nontoxic, nonflammable, non-explosive, and generally nonhazardous. Additionally, DEF is classified as a minimum risk for transportation. The fluid is mixed at a ratio of 32.5 percent formaldehyde-free low biuret urea and 67.5 percent deionized water. Heavier than diesel, exhaust fluid weighs 9.1 pounds per gallon, and while it will freeze at 12 degrees, its composition and quality are not affected by freeze or thaw.
SCR is highly effective and can reduce nitrogen oxide (NOx) emissions by as much as 90 percent or so. These systems also allow manufacturers to pump out diesel engines with huge power numbers while remaining legal.
Semi-tractors often have DEF tanks capable of holding more than 20 gallons of the fluid. Depending on the model and driving situations, that fluid may last 3,000 miles to 5,000 miles or so before needing to be refilled. Mileage varies, of course.
The math changes a little for diesel pickup trucks, but the concept is the same. For example, a current-generation Ford Super Duty has a 7.5-gallon DEF tank and, depending on use case, may go roughly 3,500 miles or so between DEF refills. Older Super Duty trucks were known for going longer between DEF fills. Towing heavy, blocky trailers reduces DEF range.
It’s also common for passenger cars with diesel engines to travel several thousand miles before needing their DEF tanks refilled.
What Happens When You Run Out Of DEF
These vehicles will warn their drivers about depleting reserves, with these warnings ranging from visual reminders to audible alarms. If you fail to replenish the DEF, or the DEF system isn’t working as designed, the truck may immediately derate or enter limp mode until the issue is resolved. To use Ford as an example here, the truck may warn that it will enter limp mode in a certain number of miles, and then slow the truck down to 50 mph once the mileage threshold is crossed. If the truck believes the issue is severe, it may eventually limit the engine to idle. Other trucks have a similar system.
Many diesel owners carry spare jugs of DEF so that they can refill anywhere when needed. I always buy extra DEF whenever I test diesel trucks because I don’t know if I will run into a fuel station that doesn’t have DEF, or run into a station that does have DEF, but the price of it is absurd.
DEF systems do have an aggravating quirk, and it’s that the derates and limp modes still occur even when a part of the system fails. Unfortunately, some manufacturers and models struggle with emissions equipment reliability more than others. One infamously unreliable diesel truck is the Nissan Titan XD Cummins, which you can read my story about by clicking here. The cruel twist is that you could have a full DEF tank, but get stuck during a road trip because of a sensor or other system failure.
The EPA Rolls Back Requirements
Back in August 2025, the EPA decided to address the issue of diesel vehicles derating after either running out of DEF or a system failure. This is what the EPA announced:
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. This ensures that bureaucratic steps do not delay manufacturers’ ability to put solutions into the field.
[…]
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.
The EPA did not completely eliminate derates and limp modes. Instead, the new rules called for a grace period. A semi-tractor operating under the new rules would be able to drive 650 miles or 10 hours before a 15 percent reduction in torque. If the DEF issue is not resolved, a 30 percent torque reduction occurs at 4,200 miles or 80 hours past the beginning of the issue. Top speed doesn’t get limited until 8,400 miles or 160 hours, and only then will the truck slow to 25 mph. If you continue to ignore the issue with the truck in 25 mph limp mode, it’ll finally derate to 5 mph.
The timing is different for other diesel vehicles. If you have a heavy-duty diesel pickup or a diesel car, the vehicle will enter limp mode 4,200 miles or 80 hours after the detection of an issue. Limp mode top speed will be 45 mph. If that’s ignored, top speed drops to 5 mph.
Many diesel owners considered this a win. Basically, the new rules mean that if a DEF system takes a dump during a road trip, there’s enough buffer to get the vehicle back to home base to get it fixed. A grace period seems like a good idea, if only so you don’t get stuck in the middle of nowhere with a broken truck and whatever you’re towing.
Conversely, the EPA’s grace period doesn’t make a distinction between a system failure and simply running out of fluid. That implies that you might be able to intentionally drive around with an empty tank for a while.
EPA Takes It Further
Yesterday, the EPA announced further changes designed to complement the August rules. Specifically, the agency suggests that it wants to hold manufacturers accountable for DEF system failures. From the EPA:
The issue facing farmers, truck drivers, and equipment operators remains clear to the agency. Sudden speed losses and shutdowns caused by DEF system failures compromise safety and productivity. The Trump EPA’s guidance, which significantly reversed deratements, called on manufacturers to revise DEF system software in existing fleets to prevent these sudden shutdowns and give operators more time to repair faults. EPA recognizes that to improve systems, the product designs and materials for these systems must also be the highest quality possible. With today’s action, EPA is taking another step to address any potential insufficiencies with system parts to better advance future rulemaking and reduce system failures.
Under Section 208(a) of the Clean Air Act, EPA is authorized to require manufacturers to provide information needed to assess whether emission control systems are functioning properly and whether manufacturers are meeting their obligations to identify and fix defects. EPA is demanding information from the top 14 on-road and nonroad manufacturers that account for over 80 percent of all products used in DEF systems. Specifically, EPA is requiring data on warranty claims, failure rates, and repair information for Model Years (MY) 2016, 2019 and 2023 emission control products to determine whether ongoing DEF system failures are related to a specific generation of products. Manufacturers will have 30 days to provide the requested information; failure may result in additional inquiries or penalties.
What’s most interesting about the press release is a short statement that was tacked on the end, which says, emphasis mine:
EPA is actively working on its proposal for the reconsideration of the 2022 Heavy-Duty Engine and Vehicle NOx rule. It is being thoroughly assessed whether derates may no longer be necessary for compliance.
If the EPA were to decide that derates are no longer relevant, it would be a major change in how diesels are run in America. There’s no shortage of diesel owners who complain about the complexity and reliability of emissions systems. Many of these people simply delete their emissions systems, and many of those who leave their systems intact do so only begrudgingly.
If the EPA said that derates and limp modes were no longer needed to be in compliance, I suspect there would be quite a few trucks rolling around on empty DEF tanks. For some diesel owners, the only reason to fill the DEF tank is to prevent limp mode. Unfortunately, the EPA has not provided any further clarity for what that future could look like.
Regardless of what happens, I will be keeping my diesels stock. I have accepted that the emissions equipment is part of the diesel ownership experience. Besides, I like the fact that I can still enjoy diesel power, but that diesel exhaust doesn’t stink and has gone through some heavy cleaning.
We await hearing more about how the EPA plans to change diesel regulations, as we may be entering a new era for diesel emissions. We will continue to watch the EPA’s developments and write updates as they come.
Topshot graphic images: Ram/BlueDEF
This is going to be unpopular, but I’m just not that sympathetic. Why should my lungs bear the cost of your luxury product? I struggle to think of a consumer application where a gas truck doesn’t achieve the same end result. And for the work trucks, DEF isn’t something new; it’s been worked into the price of goods for years now. It’s just the cost of doing business. There are solutions to these problems, but the solution is not spewing diesel fumes back into the air we breathe.
Not unpopular with me, I agree 100%.