Vehicles sold in the United States and Europe are equipped with a specific set of legal diagnostics, called on-board diagnostics; these are used to monitor the performance of various elements of the emission control system. The driver is informed of any failures by the use of a Check Engine Light on the dashboard of the vehicle and then should return the vehicle to the dealership for rectification. The vehicle manufacturer's aim is to ensure that the Check Engine Light is only illuminated for legitimate failures. For the calibration of an on-board diagnostics, there needs to be sufficient separation between the response of a good sensor and a failed sensor; the setting of this threshold should be based upon a statistical model of the data so that the predicted failures rate can then be determined. An additional benefit of obtaining a statistical model is that it allows confidence limits to be applied to the data; this then gives the engineer the ability to determine the trade-off between the number of data points and the confidence in the estimated statistical model.
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