The closed-loop emission control systems now on board vehicles depend on the measurement of oxygen by zirconia sensors. Therefore engine control is not based on direct detection of emissions but, instead, depends on a switch-like response to the air-fuel ratio. In this report, we show that an infrared diode laser which senses exhaust carbon monoxide levels can be used for closed-loop engine control. The dynamics of each subsystem of the laser CO sensor control loop were characterized. For a given speed-load point, the laser signal was found to be a smooth function of air-fuel ratio in the range of 14.0 to 14.9, in contrast with the ZrO2 sensor which operates like a rich-lean switch. Closed-loop control of the converter inlet CO level was realized at two speed-load points, one at 1200 r/min and the other 1800 r/min, and at rich, stoichiometric, or lean set-points. Closed-loop control of converter outlet CO was also demonstrated. The laser CO sensor offers the advantages of being able to eliminate limit cycle oscillations and allowing the air-fuel ratio to be set anywhere in the range from 14.0 to 14.9.