Abstract: A method to determine a performance characteristic of an exhaust system of an engine, where the exhaust system includes an exhaust gas sensor that outputs an exhaust gas signal. A combustion event in a cylinder is fueled at an air-fuel ratio selected to produce exhaust gas that is expected to be distinguishable by the exhaust gas sensor from exhaust gas produced by other combustion events, and used to determine the performance characteristic. The performance characteristic may be an exhaust gas transport time of the exhaust system, a sensitivity characteristic of the exhaust gas sensor, or a fueling correction value necessary to restore fueling to, for example, stoichiometry. Such a method is useful to detect physical changes in the exhaust system such as relocating the exhaust gas sensor, or detect if the exhaust gas sensor is damaged or not operating properly.