Ultra low sulfur diesel (ULSD) is diesel fuel refined to contain no more than 15 parts per million (ppm) of sulfur. That’s a 97% reduction from the older standard of 500 ppm, and it’s been the only diesel fuel legally sold for on-road use in the United States since 2010. The shift wasn’t just about cleaner fuel. It was designed as a package deal: engines equipped with advanced exhaust-cleaning technology that only works when sulfur is nearly absent from the fuel.
Why Sulfur Had to Go
Sulfur in diesel fuel creates sulfur dioxide when burned, which contributes to acid rain and fine particulate matter in the air. But the bigger problem is what sulfur does to modern emissions equipment. Diesel particulate filters, the devices that trap soot before it leaves the tailpipe, can achieve 90% or greater reductions in particulate matter. Sulfur poisons them. It reacts with the filter’s materials to form sulfate compounds that coat the catalyst surfaces, blocking the chemical reactions that clean exhaust gases. The same problem affects the catalytic systems that reduce nitrogen oxide emissions. Even modest sulfur levels cause these components to lose effectiveness over time.
Removing sulfur from the fuel was the only practical solution. Fuel additives don’t fix the problem because they can’t change the underlying chemistry of how sulfur interacts with exhaust treatment systems. The EPA recognized this and built its emissions strategy around treating the engine and fuel as a single integrated system.
How Refineries Remove the Sulfur
Crude oil naturally contains sulfur compounds, and diesel fuel derived from it can have sulfur levels in the thousands of ppm before treatment. Getting that down to 15 ppm requires a refining process called hydrotreating. In a hydrotreater, diesel fuel is mixed with hydrogen gas and passed over a catalyst at high temperature and pressure. The catalyst, typically made of molybdenum supported on aluminum oxide and boosted with cobalt or nickel, accelerates chemical reactions that break sulfur atoms free from fuel molecules. Those sulfur atoms bond with hydrogen to form hydrogen sulfide gas, which is captured and removed.
Reaching the 15 ppm threshold is significantly harder than reaching older limits like 500 ppm. The last traces of sulfur are locked in complex molecular structures that resist removal, so refineries had to upgrade their hydrotreating units with higher pressures, higher temperatures, and longer processing times. Many conventional hydrotreaters needed substantial modifications to produce ULSD consistently. This added cost at the refinery level, which initially raised diesel prices by a few cents per gallon during the transition period.
The Regulatory Timeline
The EPA first imposed limits on diesel fuel sulfur content in 1990 to help buses and trucks meet emission standards that were rolling out. But the transformative regulation came in 2006, when the agency required refineries to begin producing ULSD at 15 ppm for highway vehicles. By June 2010, all on-road diesel sold in the U.S. had to meet the 15 ppm standard.
Nonroad diesel followed a similar path. In 2004, the EPA finalized standards for construction, agricultural, and industrial diesel engines that would cut their emissions by more than 90%. Because those engines also needed clean exhaust technology, the rule lowered allowable sulfur in nonroad diesel fuel by more than 99% on a phased schedule. Locomotive and marine diesel followed with their own timelines.
Outside the U.S., the European Union and Japan also adopted 10 to 15 ppm standards. Mexico moved to ULSD nationwide in 2009, while Chile and Brazil mandated it in urban areas between 2009 and 2013. In much of the developing world, however, diesel with 50 ppm or higher sulfur content remains the norm.
Effects on Older Diesel Engines
ULSD works in any diesel engine regardless of age, but the transition wasn’t entirely seamless for older vehicles. The hydrotreating process that strips sulfur also reduces the aromatic content of the fuel. Aromatics are compounds that cause certain rubber and elastomer seals to swell slightly, which in older fuel systems actually helps maintain a tight seal. When those aromatics drop, seals can shrink and begin to leak.
This showed up most visibly in fuel injection pump leaks, particularly in older Volkswagen TDI engines and other European diesels from the pre-2007 era. Some owners reported that switching abruptly from biodiesel blends (which are high in aromatics and swell seals) to straight ULSD made the problem worse. The issue is related to the fuel’s aromatic and solvent properties rather than the sulfur content itself, but it’s a direct consequence of the refining process used to reach 15 ppm.
For most modern diesel vehicles built after 2007, this isn’t a concern. Their fuel systems were designed with ULSD-compatible seal materials from the start.
Lubricity and Fuel Additives
Sulfur compounds in diesel naturally provide some lubrication to fuel system components like injector pumps and fuel injectors. Removing them reduces the fuel’s lubricity, which can increase wear on precision-machined parts. Refiners and fuel distributors compensate for this by adding lubricity-enhancing additives to ULSD before it reaches the pump. The ASTM D975 fuel specification includes a minimum lubricity requirement that ULSD must meet, so the fuel you buy is already treated.
If you’re running older equipment or want extra protection, aftermarket diesel fuel additives that boost lubricity are widely available. They’re inexpensive and commonly used in the trucking industry.
Storage Considerations
ULSD behaves differently in storage tanks compared to older high-sulfur diesel. The lower sulfur and aromatic content makes ULSD somewhat more susceptible to water absorption. Water in a fuel tank creates the conditions for microbial growth, the “diesel bug” that produces slimy biofilms and clogs filters. High-sulfur diesel had a mild natural biocidal effect because sulfur compounds are toxic to many microorganisms. With that gone, microbial contamination becomes more of a management issue.
This matters most for fuel stored for long periods, such as in backup generator tanks, seasonal equipment, or marine applications. Keeping tanks full to minimize condensation, using water-separating filters, and periodically testing for microbial contamination are standard practices. Biocide treatments are available when contamination is detected. For vehicles that burn through fuel regularly, storage stability is rarely a problem.
How to Identify ULSD at the Pump
In the U.S., all diesel fuel sold at retail pumps for on-road vehicles is ULSD. Pumps are required to be labeled, and you’ll typically see a sticker reading “Ultra Low Sulfur Highway Diesel Fuel (15 ppm Sulfur Maximum).” If you’re purchasing diesel for off-road use, check the label carefully, as some nonroad fuel in certain regions may still carry higher sulfur levels depending on the application. The dye color also differs: on-road ULSD is undyed (clear or slightly amber), while off-road diesel is dyed red to indicate it hasn’t been taxed for highway use, though it may still be ULSD in composition.