A study was undertaken to evaluate the feasibility of using the catalyst exotherm to diagnose the emission performance of the catalytic converter. The exotherm was evaluated as a potential diagnostic for large volume underfloor converters as well as for small volume warmup converters. Emphasis was placed on the ability to properly diagnose the emission performance of the converters while the vehicle was driven under a variety of transient driving schedules. For this study, type K thermocouples were used for measuring the temperatures.To minimize the variability of the exotherm data during transient driving, the exotherm needs to be sampled under fairly stable exhaust flow conditions. If a transient maneuver such as an acceleration occurs, a stabilization time is required before the exotherm can be sampled.The steady-state HC conversion of underfloor catalytic converters correlated well with the exotherm measured at the rear of the catalyst over a large range of conversions. However, due to the large thermal mass and consequent long stabilization times of underfloor converters, it would be difficult to use the exotherm to diagnose the steady-state HC conversion during transient driving.For a large range of FTP hydrocarbon emissions, the FTP HC performance of underfloor converters correlated well with an equation that utilized exotherms measured at both the front and the rear of the catalyst. The front and rear exotherms were intended to reflect the lightoff performance and the warmed-up performance of the catalyst, respectively. However, similar to the steady-state diagnostic, the time required for the rear exotherm to stabilize after a transient maneuver would make it difficult to use these exotherms to diagnose the FTP performance of underfloor converters during transient driving.Since most of the initial increase in FTP HC occurs due to a loss of converter lightoff performance, it may be feasible to monitor small increases in the FTP HC for underfloor converters by monitoring the exotherm only in the front part of the catalyst. Stabilization criteria for the vehicle speed and the exotherm were developed that significantly reduced the variability of the sampled exotherm data (similar criteria could be used for the exhaust flow rate and the exotherm). The sampled exotherms decreased with increasing vehicle speed. The data suggest that the bed temperature sensor needs to be placed farther back than 2.54 cm from the front face of the brick, in order to more accurately diagnose catalysts that have been deactivated due to engine misfire.It may be feasible to diagnose the FTP performance of small volume warmup converters with the exotherm. This could be appropriate for TLEV, LEV, and ULEV vehicles in California, where warmup converters may be used to help meet the more stringent emission standards. For three 0.51L warmup converters with different FTP HC conversions, temperature data were collected on a variety of transient driving schedules. For converters with FTP HC conversions of 90 % and 58 %, the exotherm data collected when the stabilization criteria were satisfied were found to be significantly different at a 95 % confidence level. Again, the exotherms varied with the vehicle speed. This is attributable in part to decreasing HC, CO, and NOx emissions with increasing vehicle speed.For both underfloor converters and warmup converters, the exotherm data in a certain speed interval (e.g., 30-40 MPH) could be processed with on-line statistical techniques (e.g., Exponentially Weighted Moving Average). The result of this EWMA analysis could be compared to a threshold value for that speed interval stored in the control module. Similar analysis could be performed for the exotherm data in other speed intervals. The results of the analyses for the different speed intervals could be used collectively to determine if the Malfunction- Indicator-Light should be illuminated.