Cars are highly dependent on catalyst-based aftertreatment technology today due to tighter exhaust emission regulations. Since catalyst performance degradation over long-term operation is a major concern, onboard diagnostic (OBD) technology is becoming increasingly important. This paper presents new three-way catalyst diagnostic methods that can be applied under various driving conditions. Their effectiveness in meeting OBD requirements was evaluated when applied to CNG vehicles, which will be introduced in Europe. In the conventional catalyst diagnostic routine, large variation in the air-fuel ratio is generated when performing diagnosis, and catalyst degradation is determined by evaluating the signal response of an O2 sensor installed downstream of the catalyst. However, this method can only be used in conditions close to steady-state operation. In real-world driving, where a variety of operating conditions are possible, the rate of catalyst monitoring may decrease. Therefore, a different catalyst degradation diagnostic method is needed that can be used under air-fuel ratio conditions similar to those of real-world driving. One of the devised methods described here is to calculate a degradation index IDX using the formula below. where, σF and σR are the standard deviation of the oxygen concentration upstream and downstream of the catalyst. A larger value of IDX represents larger fluctuation. The IDX value indicates degradation of the catalyst; a brand new catalyst has a value of IDX = 1.0. As catalyst degradation progresses, a situation occurs where the fluctuation in the oxygen concentration before and after the catalyst becomes the same. This is indicated by a value of IDX = 0.0. A catalyst degradation diagnosis using this formula showed a value of IDX ≈ 0.6, verifying that this method detected catalyst degradation correctly. Moreover, the results showed that the diagnosis was performed many times. The IDX index was found to have a lower value for various catalysts when emissions deteriorated, indicating that emission performance correlates with IDX. The rate of catalyst monitoring during ordinary driving increases when using this new diagnostic technique. It also enables more advanced catalyst diagnosis that is expected to be made a global standard in the future.