A method, comprising: operating a diesel engine of a heavy-duty truck such that the diesel engine generates an exhaust gas flow that enters an exhaust after-treatment system of the heavy-duty truck, the exhaust after-treatment system including a selective catalytic reduction system (116; 126); measuring a level of NO x gases in the exhaust gas flow downstream of the selective catalytic reduction system; controlling a diesel exhaust fluid injector (110; 124) upstream of the selective catalytic reduction system to inject diesel exhaust fluid into the exhaust gas flow upstream of the selective catalytic reduction system at an injection rate that is based on the measured level of NO x gases; determining whether ammonia slip is occurring at the selective catalytic reduction system, including: adjusting the injection rate, thereby changing an ammonia-to-NO x ratio at the selective catalytic reduction system; measuring first levels of NO x gases in the exhaust gas flow upstream of the selective catalytic reduction system and second levels of NO x gases in the exhaust gas flow downstream of the selective catalytic reduction system as the ammonia-to-NO x ratio at the selective catalytic reduction system changes; and determining a measured efficiency of the selective catalytic reduction system based on the measured first and second levels of NO x gases; and adjusting (160, 162) the injection rate based on whether ammonia slip is occurring at the selective catalytic reduction system, characterized in that determining whether ammonia slip is occurring at the selective catalytic reduction system includes determining whether the measured efficiency is positively or negatively correlated with the changing ammonia-to-NO x ratio, wherein it is determined that ammonia slip is not occurring if the measured efficiency is positively correlated with the changing ammonia-to-NO x ratio and wherein it is determined that ammonia slip is occurring if the measured efficiency is negatively correlated with the changing ammonia-to-NO x ratio. operating a diesel engine of a heavy-duty truck such that the diesel engine generates an exhaust gas flow that enters an exhaust after-treatment system of the heavy-duty truck, the exhaust after-treatment system including a selective catalytic reduction system (116; 126); measuring a level of NO x gases in the exhaust gas flow downstream of the selective catalytic reduction system; controlling a diesel exhaust fluid injector (110; 124) upstream of the selective catalytic reduction system to inject diesel exhaust fluid into the exhaust gas flow upstream of the selective catalytic reduction system at an injection rate that is based on the measured level of NO x gases; determining whether ammonia slip is occurring at the selective catalytic reduction system, including: adjusting the injection rate, thereby changing an ammonia-to-NO x ratio at the selective catalytic reduction system; measuring first levels of NO x gases in the exhaust gas flow upstream of the selective catalytic reduction system and second levels of NO x gases in the exhaust gas flow downstream of the selective catalytic reduction system as the ammonia-to-NO x ratio at the selective catalytic reduction system changes; and determining a measured efficiency of the selective catalytic reduction system based on the measured first and second levels of NO x gases; and adjusting the injection rate, thereby changing an ammonia-to-NO x ratio at the selective catalytic reduction system; measuring first levels of NO x gases in the exhaust gas flow upstream of the selective catalytic reduction system and second levels of NO x gases in the exhaust gas flow downstream of the selective catalytic reduction system as the ammonia-to-NO x ratio at the selective catalytic reduction system changes; and determining a measured efficiency of the selective catalytic reduction system based on the measured first and second levels of NO x gases; and adjusting (160, 162) the injection rate based on whether ammonia slip is occurring at the selective catalytic reduction system,