NOVELTY - providing a method of detecting the function of a catalytic converter in a vehicle by receiving data collected by an emissions sensor and corresponding to measured hydrocarbon and other tailpipe emissions concentrations in an exhaust stream.    USE - For detecting the function of a catalytic converter in a vehicle.    ADVANTAGE - The invention decreases errors of commission which occur when an open path emissions sensor identifies a vehicle as having excess emissions, only to discover in a confirmatory closed-path tailpipe emissions test that the vehicle is functioning within emissions laws.    DETAILED DESCRIPTION - Detecting the function of a catalytic converter in a vehicle comprises determining a first indication corresponding to whether a vehicle moving on a road is operating in a closed loop operating condition; determining a second indication corresponding to whether the vehicle is operating at a temperature that is at least equal to its normal operating temperature; receiving data collected by an emissions sensor; and determining a third indication corresponding to whether the data is indicative of a properly functioning catalytic converter. The data corresponds to measured hydrocarbon and other tailpipe emissions concentrations in an exhaust stream of the vehicle.    INDEPENDENT CLAIMS are also included for:    (a) system for determining whether a properly functioning catalytic converter comprising an emissions sensor capable of measuring measured hydrocarbons in an exhaust stream of a moving vehicle; a processor in communication with the emissions sensor; and a memory in communication with the processor and containing computer program instructions that instruct the processor to perform the detection method; and    (b) a catalytic converter function detection system comprising a mechanism for determining a first indication corresponding to whether a vehicle moving on a road is operating in a closed loop operating condition; a mechanism for determining a second indication corresponding to whether the vehicle is operating at a temperature that is at least equal to its normal operating temperature; a mechanism for receiving data collected by an open path emissions sensor; and a mechanism for determining a third indication corresponding to whether the data is indicative of a properly functioning catalytic converter.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram of the method of detecting a catalytic converter function.    Assessing catalytic converter function begins (10)    Determining load to which an engine of a vehicle is being subjected (12)    Operating at normal operating temperature (14)    Determining tailpipe signature (16)    Determining whether vehicle's load is exceeding predetermined threshold (18)    Fuel enrichment event (20, 24)