There are three primary parameters that must be controlled in an engine for optimum performance; air, fuel (A/F ratio), and ignition (spark). Similarly, there are three primary parameters that must be controlled in a three-way catalyst for optimum performance; air, fuel (A/F ratio), and energy (temperature). Currently, most engine management systems provide closed loop control on the A/F ratio via oxygen sensor feedback, but temperature control is left to open loop methods with models and calculations. This paper will describe the benefits of closed loop catalyst temperature control (directly measured) for reducing emissions and providing a more robust catalyst monitor function. This catalyst monitor function will include measurement of catalyst lightoff performance which is the dominant effect causing HC tailpipe emissions to exceed OBD threshold levels. The lightoff measurement is not effected by driving cycle. This system is particularly applicable for ULEV systems applying small volume lightoff catalysts. Additionally, this work presents a system where the emission reduction system and the OBD catalyst monitor function are not in conflict with regard to optimum performance.