Method for improved performance of an engine emission control system

An engine control system of an internal combustion engine coupled to a NOx trap (72) uses a integrated difference between NOx exiting the trap (72) and NOx entering the trap (72) to determine NOx storage capacity. NOx exiting the trap is measured by a NOx sensor downstream of the trap. NOx entering the trap (72) is determined based on engine operating conditions. The values are integrated until the ratio of NOx exiting to NOx entering reaches a predetermined threshold.