TMC Survey Results Indicate Diesel Exhaust Fluid (DEF) Misfueling Remains a Costly Maintenance Concern

ABSTRACT

During the Fleet Talk session at the Technology & Maintenance Council’s (TMC) 2026 Annual Meeting, several fleet attendees shared reports of diesel exhaust fluid (DEF) contamination/misfueling with diesel. Accordingly, TMC staff conducted a survey of its fleet executive level members to determine if similar issues have been experienced by a broader cross-section of fleets. The results appear to indicate a majority of those fleet representatives responding did experience at least one case of DEF misfueling in an onboard vehicle diesel tank (12 of 21; about 57 percent.)

The incidents can apparently prove costly, with most reporting experiencing repair expenses of $10,000 or less. However, 13 such reported cases cost the fleet between $10,001 and $30,000. One additional reported case cost between $30,001 and $40,000.
While the survey response rate was not large (about 4.5 percent), the results do appear to confirm that DEF misfueling is an issue even 16 years after the introduction of downstream exhaust aftertreatment starting with the 2010 model year engines. Given this, it may be worthwhile for TMC to study the issue in greater depth and charge appropriate Council study groups with developing recommendations on how to prevent, minimize and manage the consequences of DEF misfueling.