1. A method for monitoring emissions in an internal combustion engine that emits noxious pollutants and provides a plurality of attached sensors for measuring select parameters of the engine operation as sensor output values, comprising the steps of: storing in a predictive model a representation of the combined engine and a standard external emissions monitor, the standard external emissions monitor operable to measure the noxious pollutants output by the engine, the predictive model having an output that provides a prediction of the output of the standard external emissions monitor as a predicted emissions value and operable to receive as inputs select ones of the sensor output values; inputting the select ones of the sensor output values to the predictive model; and predicting the output of the combined engine and external emissions monitor to provide an indication of the noxious pollutants output by the emissions monitor without requiring the standard external emissions monitor to be present on the engine.