Abstract: An oxygen storage capacity (OSC) monitoring system for a vehicle having a catalytic converter includes an inlet oxygen sensor that generates an inlet sensor signal (ISS) based on an oxygen content of exhaust flowing into the catalytic converter. A control module receives the ISS, increases a closed loop fuel control gain during a first period and determines a fuel control factor based on the ISS during the first period. The control module determines an OSC when an average value of the fuel control factor is greater than a first value and is less than a second value during the first period.