The U.S. Environmental Protection Agency issued a guidance letter late last year to heavy-duty on-highway engine manufacturers outlining how it intends to determine the physical range of adjustment of diesel exhaust fluid quality for certification testing, according to

Because operator intervention is needed to refill DEF, the letter says there is potential to add liquid other than DEF, either accidentally or intentionally. The agency says a financial motive could also exist to refill the DEF tank with other liquids, as well as diluted DEF.

At the time of EPA certification of the manufacturer's engine, the agency says it examine what means the engine maker has implemented inhibit DEF quality adjustment. It notes that sensors in current and previous model years have been able to detect poor DEF quality for many engines, but not all, DEF dilution scenarios.

“EPA expects that operators that would tamper with DEF quality would most commonly attempt to do so by diluting DEF with water. Dilution of DEF with water can be accomplished cheaply and easily…” the letter says. “This type of dilution would cause little to no immediate damage to the [engine’s] selective catalytic reduction system and would not affect performance characteristics apparent to the operator, such as developed power or fuel economy, though it would likely lead to a substantial increase in nitrogen oxide emissions."

EPA says using the cost range for DEF of $3 to $5 per gallon and assuming 25% dilution with water, an operator that drives 100,000 miles a year, achieves a fuel economy of 6 miles per gallon, and whose engine doses DEF at 3% of its fuel consumption rate, could save from $375 to $625 per year in DEF costs.

The incorporation of DEF quality sensors could be a suitable option and the EPA believes that urea quality sensors can be installed on new vehicles by 2016.

A copy of the letter is on the EPA website