Control device for an internal combustion engine

摘要

Prior to failure determination of a three-way catalyst, which is carried out on the basis of a reverse frequency ratio of upstream and downstream O₂ sensors, deterioration determination is performed at the point when the reverse frequency ratio of the O₂ sensors starts to increase, and low NOx control is then implemented. The implement of the low NOx control makes it possible to compensate for the deterioration of the three-way catalyst with respect to NOx purification and to keep NOx emissions from the three-way catalyst equal to or less than the regulation value.