Abstract: A method for determining the effectiveness of a catalyst having both first, relatively high oxidizable material provided to remove emissions from the exhaust of an internal combustion engine and a second, relatively low oxidizable material provided to remove emissions from such exhaust. The method includes measure a difference in oxygen content upstream and downstream of the catalyst while the engine is producing the exhaust to determine the effectiveness of the first material and determining the effectiveness of the second material by comparing time delay in a property of the exhaust as such exhaust passes through the catalyst. In one embodiment, the property of the exhaust is the oxygen content in such exhaust. In one embodiment, the effectiveness of the second material is measured after the first material is determined to be ineffective.