This explanation was plagiarized from "Bosch Fuel Injection & Engine Management" By Charles O. Pobst, SAE. Pages 2-18, 5.1, Lambda Sensor............"the three way catalytic converter works best when the air-fuel ratio is near stoichiometric, when lambda (the excess air factor) is within a very narrow range around lambda=1. Fuel injection systems, while very good at controlling the air-fuel ratio, cannot hold the air-fuel mixture within the required range. The necessary precision requires the additional feedback available from a closed-loop control system. The source of this feedback is a lambda sensor (also called an exhaust gas sensor, or oxygen sensor), installed in the exhaust system. The sensor generates a low voltage signal; the signal's strength is based on the amount of unused oxygen remaining in the exhaust stream-an indirect measurement of the air-fuel ratio. The lambda sensor signal provides feedback to the fuel injection ECU, indicating by actual results whether the air-fuel ratio needs to be corrected. The system can then adjust its fuel metering so that lambda=1, and the exhaust remains as clean as possible."