PROBLEM TO BE SOLVED: To provide a new abnormality determination system of an ozone reducing function which can determine the presence or absence of tampering of a vehicle component regardless of the driving state of a vehicle.SOLUTION: As shown in Figure 3, in every test radiator, the rate of pressure change rises with increase in coating amount. The pressure change is caused by the narrowing of a distance between adjacent louver fins due to the coating of the core of a test radiator with an ozone reducing material. The rate of pressure change becomes higher as the coating amount is increased because the distance between the louver fins becomes narrower as the coating amount is increased, making it more difficult for atmospheric air to pass therethrough. Thus, based on the relationship of Figure 3, a characteristic in the rate of pressure change (reference characteristic) is previously determined according to coating amounts and catalyst specifications. The reference characteristic is compared with a characteristic (measured characteristic) in the rate of pressure change measured during the driving of a vehicle 10, thereby determining the presence or absence of tampering.