Air/fuel ratio control method for a lean burn combustion engine

An engine control system of an internal combustion engine coupled to a NOx trap (72) uses a measured air-fuel ratio downstream of the NOx trap (72) to control engine air-fuel ratio. The measured air-fuel is only used for feedback when it indicates a value away from stoichiometry. Thus, NOx and oxygen storage effects that cause inlet air-fuel ratio to be different from exit air-fuel ratio do not cause degraded feedback air-fuel ratio control.