Oxygen Sensor 1 (O2S-1)

The O2S-1 is located in the exhaust manifold used by the PCM to make fuel control corrections toward a 14.7 to 1 air/fuel ratio. The O2S-1 is an electrical source that responds to oxygen content in the exhaust manifold. When the sensor reaches approximately 316°C (600°F), it produces a voltage based on the difference in oxygen between the atmosphere and exhaust gas. The PCM sends a bias voltage (391 - 491 mV) on the signal line which can be read on the Scan tool when the sensor is cold. When the O2S-1 is cold, it produces no voltage and has extremely high internal resistance. However, when the sensor heats up, it produces voltage that overrides the bias voltage. This voltage is read by the PCM to determine a rich/lean O2S-1 signal used to adjust injector pulse width. Under normal conditions, low sensor voltage means high oxygen content/lean air-fuel mixture and vice versa. Normal sensor readings will fluctuate between 10 mV and 999 mV.