Engine: Oxygen Sensors (O2)

At cold engine start-up the O2 sensor output will show rich (voltage near 1.0) then probably switch to lean (voltage near 0.0) and slowly move toward the rich end. As the engine warms, usually above 168° F coolant temp, the voltage will quickly start to cycle (low to high to low to high, etc.). This is absolutely normal, as the ECU is using the O2 sensor voltage in a closed-loop feedback to increase a little then decrease a little the fuel injector pulse width in an attempt to achieve the ideal 14.7:1 mixing ratio. As the engine RPM slowly increase, the cycling rate of the lights will also increase. Compare at idle and at steady 3000 RPM with the car parked. If the engine RPM increase rapidly (car stationary) or the car is accelerating, the O2 voltage will indicate a rich condition (voltage above 0.8 or so). When the throttle is closed rapidly, the O2 voltage will show lean (voltage at or near 0) because the injectors are being shut off by the ECU. At low-load cruising and light acceleration, the voltage should cycle low-high-low-high, etc.