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To meet US EPA light-duty vehicle emission standards, the vehicle powertrain has to be optimally controlled in addition to maintaining very high catalyst system efficiency. If vehicles are operated outside the bounds of a standard laboratory exhaust emission test (e.g., on-road or off-cycle) the operating control strategy may shift to optimize other desirable parameters such as fuel economy and drivability. Under these circumstances. The engine control system could be operating in a different state space from an emission control stand point. This control state-space can be observed based on four principal parameters: NOx, Lambda and exhaust temperature (measured at the tailpipe) and vehicle acceleration. These vehicle emission control patterns can be characterized by their corresponding emission control signatures, such as cold start, transient fuel control, and high speed/high load open loop. These emission control signatures are unique to a variety of engine technologies as well.