Patent ID: 8893475

Claim:
A method, comprising: determining whether selective catalytic reduction (SCR) test conditions are present; in response to the SCR test conditions being present, operating an SCR aftertreatment system at by injecting a reductant at a plurality of reductant amounts to achieve a plurality of ammonia to NOx ratio (ANR) operating points having an ANR less than 1; determining a deNO x efficiency value corresponding to each of the plurality of ANR operating points; in response to the deNO x efficiency values corresponding to each of the plurality of ANR operating points, determining a reductant correction value in response to a difference between a realized deNO x efficiency change over the plurality of ANR operating points and an expected deNO x efficiency change; and providing a reductant injection command in response to the reductant correction value; wherein the operating the SCR aftertreatment system at a plurality of reduced ammonia to NOx ratio (ANR) operating points comprises operating the SCR aftertreatment system at a first test ANR value that is lower than 0.3 and at a second test ANR value that is greater than 0.6, and the determining the reductant correction value comprises determining a test slope in response to the first test ANR value and the second test ANR value.