In a method and system to diagnose exhaust gas sensor deterioration, an automobile includes an exhaust gas system. The exhaust gas system has a fuel delivery system, an engine, a catalytic converter, an exhaust gas sensor, a processor, and/or a memory. The fuel delivery system supplies fuel which is combined with air to form a mixture combusted by the engine to generate exhaust. The processor controls the air-fuel ratio of the mixture based on the air-fuel ratio data of the exhaust detected by the exhaust gas sensor. The catalytic converter reduces pollutants in the exhaust. The processor analyzes the air-fuel ratio data to determine one or more time constants of a step response for rich to lean fuel operations and/or lean to rich fuel operations. The processor compares the time constants to a predetermined time constant threshold to determine if the exhaust gas sensor has malfunctioned.