On Tuesday, the California Air Resources Board (CARB) announced that its real-world driving tests had discovered higher-than-expected emissions levels from medium- and heavy-duty trucks with Cummins engines.

Cummins cooperated with CARB and already has ways to fix the vehicles to bring their engines' emissions numbers back in line. But the company will voluntarily recall around 500,000 trucks produced between 2010 and 2015 in order to come back into compliance with federal emissions standards.

You'd be forgiven for thinking this sounds like Volkswagen's 2015 diesel scandal . VW Group also had about 500,000 cars implicated in the US after a series of real-world tests found that its cars were producing emissions in excess of what Volkswagen reported to the Environmental Protection Agency.

But beyond surface-level comparisons, the two cases are distinct. In this case, the emissions discrepancies in Cummins engines were caused by defective catalysts rather than by unreported software, as was the case for VW Group's diesels. Catalysts are generally used to neutralize harmful emissions like nitrogen oxide (NO X ). The Cummins catalysts, CARB reports, degraded more quickly than expected, leaving vehicles on the road still operating but without a fully functioning emissions control system.

Defective hardware, not malicious intent

Because the discrepancy arose from a mechanical issue rather than due to the presence of software that is expressly prohibited by the EPA, and because Cummins cooperated with CARB from the outset, the Indiana-based engine company has spared itself some significant costs. The recall is voluntary and will involve fixes that CARB and the EPA have already approved (by contrast, it took years for VW Group to get some of its fixes for some of its diesels approved).

Interestingly, however, Cummins' emissions issues might never have been caught were it not for the VW Group diesel debacle. In 2016, in the wake of the diesel scandal, CARB started testing older vehicles under real-world driving conditions rather than simply testing newer vehicles. Using a Portable Emissions Monitoring System, the regulatory agency was able to discover the issue in the Cummins engine family.

"The testing confirmed that the selective catalytic reduction (SCR) systems were defective, causing emissions of NO X to exceed state and federal standards," a CARB press release noted. "The same problem was found to affect about 60 'engine families' under the Cummins name found in a wide range of vehicles, from big-rigs, to larger pickup trucks and some buses."

According to Reuters, a separate recall is also underway involving "Cummins engines in around 232,000 Dodge Ram 2500 and 3500 pickup trucks made by Fiat Chrysler." The EPA opened an investigation into these trucks in 2017.

In an email to Ars, Katie Zarich, an external communications manager for Cummins, wrote, "Our engines were designed to meet the emissions regulations, and we had a component challenge involving degradation of the SCR at varying rates and levels causing some of the products to produce higher emissions. We changed that component in our current products and they are operating as intended and meeting the standards."