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

An engine control system for managing lean operation of an internal combustion engine coupled to a lean NOx trap (72) uses an indication of vehicle activity. A target amount of tailpipe emission per distance is adjusted based on the vehicle activity indication. Then, engine air-fuel ratio is controlled based on the adjusted target amount of tailpipe emission per distance. Also, the indication of vehicle activity is used to enable decontamination cycles of the lean NOx trap.