Diesel-engine giant Cummins agreed this week to the largest diesel recall in the history of the EPA, according to a Reuters report.

The recall affects about 500,000 medium- and heavy-duty diesel engines in vehicles produced between 2010 and 2015.

The engines were installed in everything from big rigs to so-called "vocational trucks," such as garbage and fire trucks, to RVs and large pickups.

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The engines' selective catalytic reduction systems were found to be less durable than the law requires and will be replaced.

SCR is a system that injects a urea solution, known as diesel exhaust fluid, to reduce emissions of nitrogen oxides.

The California Air Resources Board discovered the problem in its emissions compliance testing alongside Cummins, according to Reuters.

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California and federal law require that emissions control devices on all cars continue to be operational as the cars age.

Both the EPA and CARB conduct follow-up testing of used cars to ensure their engines comply with emissions requirements.

The premature wear of Cummins emissions equipment disqualified those engines. When that testing uncovered the problem with Cummins' medium- and heavy-duty engines, Cummins followed up with its own testing to confirm the problem and issued the voluntary recall.

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“This is the way it’s supposed to work. Our follow-up testing seeks to make sure that pollution controls work throughout an engine’s useful life. And, if they don’t, then companies step up to set things right,” EPA Assistant Administrator for Air and Radiation Bill Wehrum said in a statement.

The recall does not involve a cheat device, such as Volkswagen admitted to using on its TDI models from 2009 to 2015.

Cummins is currently conducting a similar recall for engines installed in 232,000 heavy-duty Ram pickups made by Fiat Chrysler. Neither recall should be confused with the recall of light-duty Ram 1500 trucks which used diesel engines from Fiat subsidiary VM Motori.