Catalyst Deterioration System Uses Existing NOx Sensors to Determine SCR Function: To comply with increasingly stringent OBD regulations, monitoring systems have been developed to detect malfunctions of various emissions control devices on a vehicle. For a vehicle equipped with a urea SCR system, it is a challenge to develop a monitoring method to detect deterioration of the catalyst. A technique was developed at Toyota to detect deterioration of the SCR catalyst with a relatively simple approach using existing sensors on the engine. According to the preliminary investigation, they determined that the method using NOx sensors was most appropriate considering complexity, accuracy, and cost. Subsequently, technology was developed to enable detection of the SCR catalyst deterioration under various engine operating conditions.