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Upstream O2 Sensor Voltage Shorted to Ground

Name of Code:
Upstream 02S Voltage Shorted To Ground

When Monitored:
With the ignition key off, after the oxygen sensor cools down: and after the next key on -- provided that it is a cold start (coolant temperature less than 100.4°F) and ambient temperature within 7.4 degrees of coolant temperature.

Set Condition:
The upstream oxygen sensor signal voltage is below 0.156 volt prior to O2 sensor heater test (cool down period, and O2 voltage less than 0.156 volt after the key on in the subsequent start.

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
- Dirt/moisture/grease causing voltage tracking in connector
- O2 sensor failure
- Powertrain control module failure
- Connector terminals
- Connector wires