See Figures 1, 2 and 3
There are two primary modes of operation of the Computerized Emission Control (CEC) feedback system: open loop and closed loop. The system will be in the open loop mode of operation (or a variation of it) whenever the engine operating conditions do not meet the programmed criteria for closed loop operation. During open loop operation, the air/fuel mixture is maintained at a programmed ratio that is dependent on the type of engine operation involved. The oxygen sensor data is not accepted by the system during this mode of operation.
When all input data meets the programmed criteria for closed loop operation, the exhaust gas oxygen content signal from the oxygen sensor is accepted by the computer. This results in an air/fuel mixture that will be optimum for the engine operating condition and also will correct any pre-existing mixture condition which is too lean or too rich.
A high oxygen content in the exhaust gas indicates a lean air/fuel mixture. A low oxygen content indicates a rich air/fuel mixture. The optimum air/fuel mixture ratio is 14.7:1.