Abstract: A system for the control of the air-fuel ratio in an internal combustion engine incorporates an electronic control unit, a sensor of exhaust emissions and a valve for metering fuel with air to control the air-fuel ratio. The electronic control unit provides for the comparison of successive measurements of the sensor output voltage under conditions wherein the fuel valve is being operated for ever increasing richness or leaness until such time as the differential measurement drops below a predetermined amount. An offset voltage is then subtracted from or added to this voltage to calculate an operating set point voltage. Thereby, the system's accuracy is maintained through the compensation for changed sensor characteristics with aging.