A system and method for evaluating the efficiency of an oxidation catalyst of a vehicle exhaust system includes a diagnostic control system including a controller operatively connected to the exhaust system and at least one temperature sensor disposed proximate the oxidation catalyst to measure the temperature of the exhaust gas. The controller determines an operating state of at least one of the engine and exhaust system and initiates a particulate filter regeneration process in response to the detected operating state. The controller defines a first diagnostic index value and calculates a second diagnostic index value based upon the first diagnostic index value and an offset value. An oxidation catalyst efficiency validation module determines the efficiency of the oxidation catalyst.