Fuel prices may rise and fall, but one of the biggest opportunities to reduce diesel fuel costs may already be sitting in your fleet. A new whitepaper from Questar Auto Technologies found that mechanically degraded diesel aftertreatment systems can increase fuel consumption by an average of $25 to $30 per vehicle, per day. For fleets with multiple affected vehicles, those losses can quickly add up to tens of thousands of dollars annually.

While many fleets closely monitor fuel prices, route efficiency, and driver behavior, aftertreatment maintenance may be one of the easiest opportunities to improve fuel economy without changing daily operations.

The Opportunity May Be Bigger for Work Truck Fleets

For over-the-road trucking fleets, diesel particulate filters (DPFs), selective catalytic reduction (SCR) systems, and regeneration cycles are familiar territory. Drivers and maintenance teams are generally trained to recognize warning lights and understand the consequences of ignoring them.

Vocational fleets often operate differently. Many diesel pickups, vans, and work trucks are driven by technicians, contractors, landscapers, utility crews, delivery drivers, and other professionals whose primary job isn't driving. Their focus is getting to the next customer or jobsite, not monitoring aftertreatment systems. That can make it easier for warning lights to be overlooked until performance begins to suffer.

According to Questar's analysis, waiting that long could mean paying a fuel penalty every day the issue goes unresolved.

"The analysis shows how advanced agentic technology can uncover hidden economic losses inside normal fleet operations and turn them into maintenance intelligence operators can take action on," said Dr. Yuval Shalev, Questar's vice president of data science and AI and author of the white paper.

The company analyzed a mixed-class commercial fleet and found vehicles operating with degraded DPF and SCR systems were consuming an additional $25 to $30 in fuel each day. In one example highlighted in the report, a vehicle burned approximately 0.13 additional gallons of fuel per hour while idling, representing more than a 10% increase over expected consumption.

Summer Is a Good Time for a Maintenance Reminder

The findings arrive as many fleets enter the hottest months of the year, when longer idle times often become more common as drivers cool vehicle cabins between stops. Excessive idling has long been associated with higher fuel consumption, but degraded aftertreatment systems can amplify those losses.

Summer also brings another operational concern. Vehicles left running and unattended while drivers make deliveries or complete service calls may be more vulnerable to theft. Encouraging drivers to minimize unnecessary idling not only helps reduce fuel consumption but can also lower security risks.

For fleet managers, the combination makes aftertreatment inspections and driver education a timely seasonal reminder.

Don't Wait for Performance Problems

One of the biggest misconceptions surrounding diesel aftertreatment systems is that repairs can wait until the vehicle begins losing power or experiencing drivability issues. Questar's analysis suggests fleets may already be paying for those problems long before performance noticeably declines.

"Fleets have long understood that mechanical degradation can increase operating costs, but until now had no way to accurately identify and quantify the cause," Shalev said. "This analysis shows how advanced agentic technology can uncover hidden economic losses inside normal fleet operations and turn them into maintenance intelligence operators can take action on."

The company says its platform analyzes thousands of vehicle-days of operational data, using hundreds of sensor inputs to compare predicted fuel consumption with actual usage. That process allows fleets to identify hidden fuel losses associated with mechanical degradation and prioritize maintenance based on measurable financial impact.

For many vocational fleets, the biggest takeaway may be much simpler than the technology behind the analysis.

Drivers should understand what aftertreatment warning lights mean, report them promptly, and avoid treating them as something that can wait until the next scheduled service. Addressing DPF and SCR issues early may help reduce fuel costs, improve reliability, and decrease the likelihood of more expensive repairs later.

Fleet managers interested in the research can download Questar's white paper, The Hidden Fuel Cost of Mechanical Degradation, from the company's website.

Get More Insights!

Want more insights? Download the full white paper from Questar Auto Technologies below or contact them here for more on how they can help transform diagnostics today. Just complete this form below to receive your PDF download!