a device for determining the abnormal degree of deterioration of a catalyst of a catalytic converter arranged in an internal combustion engine exhaust system is disclosed . the device includes first and second air - fuel ratio sensors located upstream and downstream of the catalyst . the device determines a first purification ability based on readings of the sensors during a warmup stage of the catalyst , and a second purification ability after the warmup is completed . an overall purification ability of the catalyst is determined by combining the purification ability of the catalyst during warmup and the purification ability of the catalyst after warmup , where the weight of the purification ability during warmup is less than the purification ability after warmup in the overall purification ability . if the overall purification ability of the catalyst is less than a certain amount , then the device determines that the catalyst has deteriorated .