Method for controlling air/fuel mixture in an internal combustion engine

A method and system for controlling the air/fuel ratio in an internal combustion engine (12) having a first group of cylinders and a second group of cylinders. The first group of cylinders is coupled to a catalyst (20) and at least one oxygen sensor (18,22), which provides a first feedback signal. The second group of cylinders is coupled to a catalyst (24) and a post-catalyst oxygen sensor (26), which provides a second feedback signal. A controller (202) uses the first and second feedback signals to calculate a short-term air/fuel bias value for the second group of cylinders. The controller (202) also calculates a new long-term air/fuel bias value corresponding to the current engine speed and engine. The new long-term air/fuel bias value is based on a previously-calculated long-term air/fuel bias value calculated for the same engine load and speed. A total air/fuel bias value is calculated based on the short-term air/fuel bias value and the long-term air/fuel bias value. The new long-term air/fuel bias value is stored for future calculations.