An internal combustion engine operating at a lean air/fuel ratio includes a reductant injection system configured to dispense reductant into an exhaust gas feedstream upstream of a selective catalytic reduction device. The reductant injection system includes a reductant delivery system fluidly coupled to a reductant dispensing device that is configured to dispense the reductant. A method for monitoring the reductant injection system includes commanding the reductant dispensing device to dispense reductant at a prescribed reductant flowrate, controlling the reductant delivery system to a preferred operating state, monitoring operation of the reductant delivery system and estimating a reductant flowrate as a function of the monitored operation of the reductant delivery system, and diagnosing operation of the reductant injection system as a function of the prescribed reductant flowrate and the estimated reductant flowrate.