Although one of two key bills aimed at reducing pollution from cars failed to pass on Utah’s Capitol Hill, environmentalists say the 2018 legislative session was largely a success for air quality.

HB171 — which would have doubled penalties for Utah drivers caught disabling a diesel vehicle’s emissions-control system — never made it to the Senate floor for a final vote before the session ended late Thursday.

Such illegal modifications cause vehicles to produce far more pollution than they would otherwise, allowing drivers to release plumes of black smoke from their tailpipes — a practice known as “rolling coal.”

Rep. Patrice Arent, D-Millcreek and a co-chair of the state’s Clean Air Caucus, said she believed the measure had enough support to pass, and that plans are in the works to run the bill again next year.

Diesel industry officials, who had strongly supported HB171, said they were disappointed by the bill’s failure.

“Manufacturers have made enormous investments in clean diesel technologies and are not supportive of any efforts to tamper with engines, components and emission controls for the purpose of generating excessive soot emissions,” Ezra Finkin, director of policy at the Diesel Technology Forum, said in a statement.

“While this legislation will not be moving forward,” Finkin said, “we continue to urge pickup enthusiasts in Utah to roll clean.”

Legislators approved another major air-quality measure HB101, after a similar measure failed to pass last year. Assuming Gov. Gary Herbert signs it into law, the bill will require Utah County to create a three-year pilot program for testing diesel vehicle’s emissions.

Among the several Wasatch Front counties that do not meet federal standards for small particulate pollution, Utah County is the only one that does not already require emissions tests for diesel vehicles.

Ashley Miller, a program director for the air-quality advocacy group Breathe Utah, said the emissions testing will be an important step forward.

“Right now, the most important thing is to get all diesel passenger vehicles tested,” Miller said. “That’s a really important step in catching illegal tampering.”

But it is possible to circumvent the emissions tests, Miller said. Without steeper penalties — currently Utahns caught driving an illegally modified vehicle are subject to a fine of $50 — she said she feared some residents would continue to flout the law.

Lawmakers also sent nearly $1 in additional funding to the state Division of Air Quality for additional research and staff. Arent said the $1 million was among the largest clean air appropriations in recent years.

“Overall it’s been a good year for clean air,” she said. “Particularly with appropriations, we did extremely well. We got almost everything we asked for.”

Bryce Bird, director of the Utah Division of Air Quality (DAQ), said the funds would be used to hire three additional staffers to help inspect industrial sources of pollution and to assist with research projects.

More than $50,000 of the new cash will go to air quality research that will help inform state officials’ ongoing struggle to find ways to improve air quality in Salt Lake City before a critical federal deadline.

Legislators also passed HB331, require students in driver’s education classes to complete coursework on pollution from vehicles. The measure made its way through both the Senate and the House of Representatives with only a single opposing vote — cast by Senator Margaret Dayton, R-Orem.

Bird said the new curriculum, which DAQ will help to create, will cover how vehicle emissions systems work, and how to reduce pollution by running errands all at once, avoiding idling, and buying a cleaner car.

Correction: Ezra Finkin is director of policy at the Diesel Technology Forum. A prior version of this story misstated his title.