Method for operating a lean burn internal combustion engine having an exhaust gas purification system

In a process to operate a lean-burn internal combustion engine, the exhaust system has a pre-catalytic converter discharging to a NOx storage catalytic converter. In the process, the storage activity is determined, together with temperature values for both catalytic converters to determine a pattern of correlation between the two. The values are compared with a standard sample to determine the actual condition within the pre-catalytic converter. Further in the process the NOx storage activity is determined at a temperature within a given diagnostic temperature range +/- 10K. These values are determined in conjunction with the pre-catalyst temperature and a correlation is established between the two and compared with a known standard sample. The pre-catalytic converter condition is derived by a comparison of the values measured with the sample. Further claimed is a lean-burn engine incorporating this diagnostic system.