21 Upstream O2 Sensor Shorted to Ground

NAME OF CODE:
Upstream O2 Sensor Voltage Shorted to Ground

WHEN MONITORED:
With the ignition key off, after the engine cools down.

SET CONDITION:
The upstream oxygen sensor signal voltage is below 0.156 volts prior to O2 sensor heater test.

THEORY OF OPERATION:
The upstream oxygen sensor is a voltage generating device. The PCM receives exhaust gas information from this O2 sensor. The sensor detects exhaust gas content by a galvanic reaction within the sensor that produces a voltage. After measuring the amount of oxygen in the exhaust gases, the upstream oxygen sensor tells the PCM how well its output signals are controlling the air/fuel ratio. Variations in the signal from this O2 sensor serve as air/fuel ratio indicators. Changes in the sensor signal occur because the air/fuel ratio is constantly changing. When oxygen content is low (rich mixture), the voltage signal will be approximately 1 volt. When oxygen content is high (lean mixture), the voltage signal will be low, approximately 0.1 volt.

POSSIBLE CAUSES:
- Sensor output wire shorted to another circuit
- Dirty/wet connection causing voltage tracking in connector
- O2 sensor failure
- PCM failure
- Connector terminals
- Connector wires