A diagnostic apparatus for an internal combustion engine is disclosed herein. In one embodiment, the diagnostic apparatus comprises a catalytic converter and a controller. The catalytic converter is disposed in an exhaust passage of an engine and treats an exhaust gas component in the exhaust gas. The controller is adapted to perform a number of functions. More specifically, the controller is adapted to operate the engine with an engine control parameter to increase catalyst temperature of the catalytic converter during a cold engine condition. The controller is further adapted to calculate a temperature factor indicative to temperature increase of the catalyst based on the engine control parameter. The controller is further adapted to estimate the amount of the exhaust gas component flowing out of the catalytic converter based on the temperature factor. Finally, in one embodiment, the controller is adapted to determine malfunction of the engine operation to increase catalyst temperature of the catalytic converter based on the amount of exhaust gas component flowing out of the catalytic converter. A diagnostic method is also disclosed.