An investigation conducted by the California Air Resources Board (CARB) found that more than 500,000 Cummins-powered heavy duty trucks were emitting excessive NOx emissions due to defective SCR catalysts. The problem affects various Cummins engine families used in applications ranging from Class 8 trucks to larger pickups to buses. After performing its own in-house study, Cummins agreed to issue the voluntary recall, which is the largest heavy duty truckrelated recall to date. CARB has emphasized the fact that the reason behind the excess NOx emissions is purely mechanical, and that no defeat device(s) or emissions cheating are responsible.