A system and method are provided for closed-loop monitoring of temperature within a vehicle emission control device to control air-fuel ratio (AFR) and corresponding production of exotherm to accurately maintain a desired temperature range. A temperature sensor is positioned within the emission control device to provide a direct measurement of temperature within the device. Inlet temperature can be either directly measured or estimated using a temperature model. Actual temperature within the device is estimated as a function of the inlet and measured temperatures. The estimated actual temperature is used to modify the AFR in the exhaust gases flowing through the emission control device.