Cummins Three-Way Catalyst : The Questions They Didn't Ask

Cummins has published thirteen friendly answers about its X15N three-way catalyst, covering maintenance, downtime, diesel exhaust fluid and the financial pleasures of not cleaning filters. Unfortunately, the company appears rather less enthusiastic about questions involving real-world emissions, methane leakage, catalyst ageing and independent verification. Its own engineers have published research showing why those questions matter. Meanwhile, American regulators have already given Cummins a $1.675 billion reason to appreciate the difference between a laboratory test and what comes out of an exhaust on the road. So, before we accept another reassuring emissions lecture from Columbus, perhaps somebody should ask the obvious question. Are we really trusting these fuckers again?

On 23 October 2026, Cummins published Three-Way Catalyst FAQs: What Fleets Should Know, a tidy little excursion through natural gas aftertreatment. The company’s X15N engine uses a three-way catalyst to control nitrogen oxides, hydrocarbons and carbon monoxide, without the diesel exhaust fluid, particulate filter cleaning or regeneration processes associated with modern diesel systems. For a fleet manager responsible for keeping expensive trucks on the road, that is potentially useful information. Nobody enjoys paying a technician to investigate a warning light while a loaded vehicle sits in the yard earning precisely fuck all.

However, this is Cummins, a manufacturer whose previous adventures in emissions compliance produced one of the largest environmental penalties in American history. Consequently, an article explaining how its latest emissions technology works deserves rather more scrutiny than the average promotional leaflet. The question is not whether three-way catalysts are legitimate engineering. They are. The question is whether Cummins has supplied enough information for customers to judge their performance beyond the carefully selected comforts of its own marketing.

And after thirteen questions, the most interesting ones are still sitting outside the workshop.

Thirteen Questions, One Convenient Agenda

Cummins begins with the basics. What is a three-way catalyst? How does it work? Does the X15N need diesel exhaust fluid? Can the technology reduce operating costs? What about downtime, regeneration, installation and maintenance? The article even considers return-to-base operations, where predictable routes and servicing schedules make the economics particularly attractive.

Nothing terribly outrageous there. A fleet buying a natural gas truck needs to understand the equipment, and Cummins is entitled to explain its commercial advantages. Nevertheless, something revealing happens when the article starts discussing how its own message was assembled.

The published text refers to priorities identified by Cummins subject-matter experts for communicating the X15N story. Later, it offers this rather wonderful insight into the editorial process:

“Cummins SMEs specifically recommended focusing on those factual differences rather than positioning the technology as a criticism of diesel.”

Well, isn’t that fucking interesting?

Here we are, supposedly reading answers to questions fleets should know, and the article has begun explaining the communications strategy behind those answers. It is like ordering a meal and having the waiter interrupt halfway through to explain which adjectives the restaurant’s marketing consultant recommended for the menu.

Cummins evidently wants the emphasis on simplicity, maintenance, uptime and total cost of ownership. Fair enough. Those are legitimate commercial considerations, especially when the same company also sells an enormous quantity of diesel hardware. Nobody wants a natural gas advertisement wandering across the corridor and kicking the diesel department in the bollocks.

But who decided which questions belonged in the FAQ? Why are maintenance savings repeatedly examined while independently measured emissions receive considerably less attention? Where are the figures showing what actually emerges from an X15N exhaust across different operating conditions?

The company has told customers what it wants them to know. What about what they might reasonably want to ask?

The Cummins Three-Way Catalyst: Chemistry Is Not A Sales Brochure

The Cummins three-way catalyst is designed to tackle three principal exhaust pollutants: nitrogen oxides, hydrocarbons and carbon monoxide. Precious metals inside the catalyst encourage chemical reactions that convert these substances into less harmful products, including nitrogen, water and carbon dioxide. Unlike a diesel particulate filter, the catalyst does not operate by accumulating soot and periodically burning it away.

That is established technology, and the underlying chemistry is sound. However, its effectiveness depends on maintaining suitable exhaust conditions, particularly the relationship between air and fuel. A modern natural gas engine must manage that balance carefully, using sensors and electronic controls while the engine encounters changing loads, speeds and temperatures.

Cummins knows this perfectly well. In June 2026, the company published another article, Profitable Uptime and Lower TCO for Natural Gas Aftertreatment, featuring Scott Baize, its Spark Ignition Regional Technical Platform Leader for North America. Baize explained the importance of accurate air-fuel control and described improvements to the X15N’s wide-band oxygen sensor. He also claimed the three-way catalyst was reducing NOx by more than 99%.

An impressive percentage. Yet percentages are rather cooperative little creatures when nobody asks what they are percentages of.

Does the figure describe conversion efficiency under particular conditions, or does Cummins have corresponding results demonstrating performance across the full range of real-world operating conditions? What absolute NOx emissions emerge from the tailpipe? How does performance change during cold starts, prolonged idling, acceleration, hill climbing and engine braking?

More importantly, what happens after hundreds of thousands of miles, when the catalyst has aged and the engine has spent years being driven by people rather less gentle than the laboratory technician responsible for calibrating it?

Cummins says its latest natural gas aftertreatment meets stringent 2027 EPA and California requirements. That is a meaningful claim about regulatory compliance. But certification, catalyst conversion efficiency and emissions measured during actual fleet operation are different kinds of evidence.

Why not explain all three?

Meanwhile, The Engineers Have Been Doing Some Actual Science

Here is where the story gets considerably more interesting.

On 7 April 2026, four Cummins researchers – Richa Raj, Mi-Young Kim, Grace Aigbiremolen and Anand Srinivasan – published an SAE technical paper titled Experimental and Kinetic Modeling Study of Aged Natural Gas Three-Way Catalyst Performance.

Unlike the October FAQ, this was not written to reassure a purchasing department that it could stop worrying about diesel exhaust fluid. The researchers examined the complicated chemical behaviour of an aged commercial three-way catalyst under different experimental conditions, including lean, stoichiometric and rich mixtures.

Their work explored how catalyst reactions depend on the air-fuel ratio, oxygen availability and interactions between different gases. The paper describes circumstances in which ammonia and nitrous oxide can form during nitrogen oxide reduction. It also examines reduced NO conversion under certain lean conditions and identifies NOx slip at the beginning of rich operating cycles, relevant to modelling engine-braking events.

Now we have something worth putting on the workbench.

The paper describes bench-scale experiments and modelling, rather than emissions measurements from a fleet of X15N trucks. Its findings therefore cannot be presented as evidence that the X15N is exceeding emissions limits. Nevertheless, the research establishes that catalyst behaviour under changing operating conditions requires serious investigation.

One part of Cummins is producing detailed kinetic models to understand what happens when combustion conditions change. Another is informing customers that the catalyst has no routine maintenance schedule and requires remarkably little attention.

Both statements can be true. However, shouldn’t the commercial explanation acknowledge the engineering complexity when customers are being asked to rely on the system for years?

What do those experimental findings mean for the calibration and durability validation of Cummins’ production natural gas engines? How does the X15N respond when conditions move away from the ideal air-fuel balance? What independent measurements establish whether potentially significant by-products remain adequately controlled?

And if Cummins’ own engineers are investigating the behaviour of aged catalysts under transient conditions, why wouldn’t a fleet operator want to see the corresponding real-world evidence before signing a purchase order?

The engineers are studying the complicated stuff. The marketing department is selling the absence of a filter.

There is quite a distance between those two conversations.

Maintenance-Free, Provided You Remember The Maintenance

The October FAQ repeatedly promotes the three-way catalyst’s lack of routine servicing requirements. No diesel exhaust fluid, no particulate filter cleaning, no regeneration and no scheduled catalyst replacement. Cummins describes a system designed to last the life of the vehicle.

That sounds lovely. Unfortunately, engines are not assembled from marketing promises, and even the most cooperative catalyst depends on the machinery surrounding it.

Cummins’ June article explained that X15N spark plugs generally require replacement at 60,000-mile intervals, potentially extending to 65,000 miles with a specified lubricant and maintenance arrangement. Poor ignition-system maintenance can produce misfires, while persistent misfiring can damage the catalyst itself.

The October FAQ does acknowledge the need to maintain the engine and replace spark plugs. Its separate maintenance guidance goes further, emphasising oil-change discipline, approved ignition components and attention to engine hours. Natural gas engines can be less forgiving than diesel engines when certain servicing intervals are neglected.

So what exactly does maintenance-free mean in practical ownership terms?

The catalyst might not require scheduled replacement, but what happens when another component causes it to deteriorate? How is that deterioration detected? What diagnostic information does the driver receive, and does the system provide enough warning before emissions performance becomes unacceptable?

Cummins says the catalyst is intended to last the life of the vehicle. Does that mean a defined mileage, operating-hour threshold or an engineering design target? Is the entire period covered by a particular emissions warranty, and what happens when an expensive catalyst fails outside the warranty conditions?

These are not unreasonable questions from somebody planning to operate a heavy-duty truck for years.

There is a significant difference between having nothing scheduled on the maintenance calendar and having nothing potentially expensive to worry about. The former is a servicing advantage. The latter would be a minor mechanical miracle, and Cummins has not yet advertised a truck capable of performing those.

The Methane Has Not Read The FAQ

Then there is the other rather substantial issue sitting behind the natural gas sales pitch.

Natural gas is predominantly methane.

A well-functioning three-way catalyst can reduce unburned methane escaping through the exhaust. However, it cannot repair a leak at a production facility, recover methane vented upstream or guarantee that every cubic metre travelling through the supply chain has been handled responsibly.

For that reason, a favourable tailpipe NOx result cannot establish the complete climate impact of a natural gas truck. Nitrogen oxides matter enormously for air quality and human health, but the greenhouse-gas calculation also needs to account for carbon dioxide, methane leakage and other relevant emissions throughout the fuel’s lifecycle.

Research published in February 2026 by scientists associated with Stanford University estimated greenhouse-gas emissions across the American natural gas supply chain at more than twice the corresponding EPA estimates used for comparison. Their analysis incorporated measurements and modelling across a vast network of production facilities and associated infrastructure.

The researchers were not testing Cummins trucks. They were investigating the industry supplying the fuel those trucks can burn. Yet their findings raise an obvious commercial and environmental question: how reliable are claims about the climate advantages of natural gas when upstream methane emissions remain difficult to measure accurately?

The International Council on Clean Transportation explored a related problem in January 2025. Using a mean upstream methane loss rate of 1.8%, its analysis estimated that compressed natural gas trucks delivered approximately 6% lower lifecycle greenhouse-gas emissions than comparable diesel trucks under the assumptions examined.

Six per cent is not nothing. However, it is considerably less impressive than the environmental transformation suggested by some of the industry’s cleaner-fuel imagery. The result also changes when methane losses, vehicle efficiency and fuel pathways change.

Those figures are not measurements of the X15N. They demonstrate why fleet-specific, fuel-specific lifecycle calculations matter.

So where does the X15N’s natural gas originate? What methane-loss assumptions are used when comparing its climate performance with diesel? Are the figures independently verified, and do they include methane released during extraction, processing, distribution and refuelling?

Perhaps Cummins could add those to its next FAQ.

The catalyst may do an excellent job cleaning particular gases at the tailpipe. Unfortunately, atmospheric chemistry does not accept a customer-service response explaining that the offending methane escaped three states earlier and is therefore somebody else’s fucking problem.

There Is Another Cummins FAQ. The EPA Wrote It.

Trust brings us to the part of the story that Cummins cannot simply polish away with another paragraph about uptime.

In January 2024, the United States Department of Justice, Environmental Protection Agency and California authorities announced a settlement requiring Cummins to pay $1.675 billion in civil penalties over alleged Clean Air Act violations involving Ram diesel engines.

Regulators described emissions defeat-device software affecting approximately 630,000 Ram 2500 and 3500 vehicles from model years 2013 to 2019. Additional undisclosed emissions-control software features were identified in other vehicles. The settlement also required recalls, mitigation measures, testing and enhanced compliance arrangements.

Cummins denied the allegations and did not admit wrongdoing. The settlement nevertheless became final and effective in April 2024, and the financial obligations were very real.

More interestingly, the EPA maintains its own frequently asked questions about the Cummins case.

Unlike the manufacturer’s October 2026 FAQ, the regulator’s questions address what the disputed software did, how the violations were discovered and why the consequences mattered.

The EPA explained that software in the affected diesel vehicles could produce compliant results during emissions testing while reducing the effectiveness of selective catalytic reduction during certain normal driving conditions. That difference increased NOx emissions outside the test environment.

Think about the significance of that for a moment.

American regulators had to investigate a manufacturer whose emissions-control software, according to their findings, allowed affected vehicles to satisfy laboratory requirements while behaving differently on the road. The resulting settlement imposed an enormous financial penalty and extensive corrective obligations.

The new X15N natural gas engine uses a different combustion and aftertreatment architecture from those Ram diesel applications. Nobody needs to pretend the SAE research or the earlier enforcement action proves misconduct involving the three-way catalyst.

But surely that history changes the level of scrutiny a reasonable customer might apply to the manufacturer’s subsequent emissions claims?

And the Ram case was not the entire regulatory record. In March 2024, California announced a separate $46 million settlement concerning undisclosed post-certification engine changes affecting approximately 120,000 Cummins engines. The authorities also identified roughly 2,000 engines with undisclosed auxiliary emissions-control devices that altered emissions-system behaviour and produced emissions above regulatory limits.

Again, a different matter. Another set of emissions-compliance questions. Another financial settlement.

Wouldn’t a company with that history be especially keen to demonstrate its latest technology through transparent, independently verifiable results?

Why should anybody be criticised for wanting evidence of how Cummins’ emissions systems behave outside carefully controlled conditions when those very differences formed the substance of a major enforcement case?

A manufacturer’s reputation is not a permanent certificate of compliance. Neither is its marketing department an independent testing laboratory.

Who Is Testing The Truck When Nobody Is Selling It?

Imagine being the fleet manager sitting opposite a Cummins representative.

There is a brochure on the table, a calculator showing projected maintenance savings and a discussion about fuelling infrastructure. The representative explains that the three-way catalyst avoids DEF purchases, DPF cleaning and regular regeneration. The numbers look attractive, especially if natural gas is available at favourable prices and the trucks operate predictable routes.

You can understand the appeal. Fuel bills, maintenance planning and vehicle availability are not imaginary concerns. They determine whether a transport operation makes money or spends another year negotiating with its bank manager.

However, suppose the fleet manager then asks for independently verified X15N emissions measurements from trucks operating in ordinary commercial service.

Not an attractive percentage from a presentation. Not a general assurance that the technology meets regulatory requirements. Actual measurements, with the test conditions, operating hours, mileage and emissions figures clearly identified.

Would those results be readily available?

Could the manufacturer explain what happens to NOx conversion during demanding transient operation, how catalyst ageing is validated and what protections exist against emissions-control degradation? Would customers receive sufficiently detailed information about relevant software calibrations and changes throughout the engine’s useful life?

If the fleet’s sustainability department wanted a lifecycle comparison, could Cummins supply the assumptions covering upstream methane leakage and the particular natural gas pathway being purchased?

These questions should not terrify a serious engineering company. They belong in the same commercial discussion as fuel economy, service intervals and total cost of ownership.

Indeed, the strongest possible response from Cummins would be to publish the evidence and allow independent scrutiny.

Instead, the October FAQ gives readers considerably more information about what the aftertreatment system does not require them to buy or maintain than it gives about independently measured emissions performance under demanding conditions.

Why?

Three-Way Catalyst, One-Way Conversation

The fascinating thing about the Cummins three-way catalyst story is that the underlying technology may offer genuine operational advantages. Removing DEF and DPF servicing requirements can simplify fleet maintenance. Effective catalytic conversion can substantially reduce harmful pollutants, while well-controlled natural gas combustion offers a different engineering approach from diesel.

None of that needs to be disputed merely because Cummins is the manufacturer.

But the October article asks readers to view the technology primarily through the lens of convenience. Less maintenance, fewer consumables, reduced downtime, easier installation and potentially lower operating costs. Its own references to expert messaging priorities make the commercial emphasis unusually transparent.

Meanwhile, Cummins’ engineers publish research on the complexities of aged catalyst behaviour. Independent researchers investigate methane emissions across the supply chain. Regulators maintain a substantial public record of the company’s previous emissions-compliance problems.

Those are three different sources of information, and together they demand a rather more substantial conversation than a list of things fleets no longer need to purchase.

Nobody is asking Cummins to abandon natural gas technology because of its history. The questions concern the evidence supporting its present claims, the transparency of its ongoing compliance work and the extent to which customers can independently evaluate what they are buying.

After all, if the company genuinely has developed a robust, durable and remarkably effective emissions-control system, there should be no great difficulty demonstrating that performance beyond a handful of favourable percentages and a catalogue of maintenance savings.

Cummins has answered thirteen questions.

TCAP has a few more.

How does the X15N perform across the full range of real-world driving conditions? What do independent measurements show after significant catalyst ageing? How are transient emissions and potential chemical by-products monitored? What methane assumptions support the environmental comparisons, and what can customers examine for themselves?

Above all, what has Cummins done to make its emissions assurances demonstrably more trustworthy following the regulatory failures documented in its previous settlements?

Because when a manufacturer has already paid $1.675 billion to resolve one enormous emissions case, with another $46 million settlement appearing separately on the regulatory record, the burden of convincing the public should not rest on whether its communications department can produce a particularly cheerful FAQ.

The company can explain why the catalyst does not need diesel exhaust fluid. It can explain the absence of regeneration, the lighter installation and the projected savings. All useful subjects, especially for somebody whose working day revolves around keeping trucks moving.

But the bigger question remains.

Are we really trusting these fuckers on emissions again simply because the brochure says everything is working beautifully?

Or might we reasonably insist on seeing precisely what comes out of the exhaust?

Lee Thompson – Founder, The Cummins Accountability Project

Cummins Confidential : Three-Way Catalyst – The Questions They Didn’t Ask