Rich burn internal combustion engine catalyst control

A catalyst system (200) may include a catalyst (220) and a first sensor (251) that detects contents of gases entering the catalyst and reports the contents of the gases entering the catalyst to an emissions control module (230). A second sensor (271) and a third sensor (272) may detect contents of gases exiting the catalyst and report the contents of the gases exiting the catalyst to the emissions control module. The emissions control module may determine an air-fuel ratio based on the contents of gases entering the catalyst and the contents of gases exiting the catalyst. The emissions control module may instruct an air-fuel regulator (241,242) to operate an engine using the air-fuel ratio.