Patent ID: 8631786

Claim:
A system comprising: an engine having an exhaust; an SCR catalyst coupled in the exhaust; a reductant injection system coupled to the exhaust; an ammonia sensor coupled to the exhaust downstream of the SCR catalyst; and a controller including computer readable storage medium with instructions thereon, comprising: instructions to, during a first mode, inject the reductant based on a first reading of the ammonia sensor; and instructions to, during a second mode following the first mode: generate a first exhaust air-fuel-ratio; receive a second reading of the ammonia sensor at the first exhaust air-fuel-ratio; generate a second exhaust air-fuel-ratio; receive a third reading of the ammonia sensor at the second exhaust air-fuel-ratio; determine ambient humidity based on a difference between the second and third readings of the ammonia sensor; and inject the reductant based on the first reading of the ammonia sensor, the first reading corrected for ambient humidity.