An engine self-diagnosis system capable of performing diagnosis of the light-off performance of a catalyst at a low cost and high accuracy without requiring addition or improvement of a sensor, etc. The engine self-diagnosis system comprises a unit for directly or indirectly detecting performance A of an exhaust cleaning catalyst when temperature of the catalyst is within a predetermined temperature range, and a unit for, based on the detected catalyst performance A, estimating performance B of the catalyst, which is resulted when the temperature of the catalyst is outside the predetermined temperature range.