Theory Of Operation
The wide-band O2 Sensor operates differently than traditional O2 Sensors. The wide-band O2 Sensor tip consists of two cells that provide different functions, a measurement chamber and a detection chamber with pumping capabilities. The oxygen pumping function is the ability to pump oxygen into or out of the measurement chamber depending on the level of oxygen in the measurement chamber. This function provides the wide-band sensing capabilities and is critical for proper oxygen measurement. The O2 Sensor Reference circuit provides a common bias supply to both the O2 Sensor Signal and the O2 Sensor Pump Cell Current circuits.
During normal operation, the O2 Sensor Reference voltage and O2 Sensor Signal voltage will be a fixed voltage value. The O2 Sensor Current Pump voltage will switch from approximately 0.45 volts above and below the fixed O2 Sensor Return voltage, allowing current to flow in either direction through the pump. This correlates with the pumping of oxygen into and out of the measurement chamber. On a properly operating vehicle, this happens very quickly and the voltage reading should maintain a steady 0.45 volts when taking a voltage measurement between the O2 Sensor Signal circuit and the O2 Sensor Reference circuit of the O2 Sensor with the engine running and the O2 Sensor operating in closed loop.
When the exhaust stream has a lean air/fuel ratio (high oxygen content) the pumping element voltage will move toward +0.45 volts pumping oxygen out of the measurement chamber. When the exhaust stream has a rich air/fuel ratio (relatively low oxygen content) the pumping element voltage will move toward -0.45 volts pumping oxygen into the measurement chamber.
The PCM is able to pinpoint failures to the specific O2 Sensor circuits through a series of steps which include both "cold" and "hot" tests. To ensure that the correct malfunctioning circuit is identified, the diagnostics are performed in three separate test conditions, a Cold test at startup, Hot test during vehicle operation and Cold test during PCM power down .
A Cold test, performed at startup (before the O2 system enters closed loop operation), can detect a Short Circuit to Battery. Although the diagnostics can detect the failure, it cannot identify the specific circuit which failed and will set the generic fault code P0132.
The Cold test at startup cannot detect a Short Circuit to Ground or an Open Circuit failure.
When the PCM detects a circuit that falls outside the allowable range during either the cold startup test or during O2 Sensor closed loop operation, a Hot test is initiated. The O2 Sensor is turned off and a 20 second delay timer is started. Once the timer expires, the hot test diagnostic will be performed. It takes approximately 160ms from the time the hot test is initiated until the fault is set. If an open circuit is detected the specific circuit which failed can be identified during the hot test and the fault code for the relating circuit will be set. If a short to ground or short to battery is detected the diagnostics can detect the failure, however it cannot identify the specific circuit which failed and will set the generic fault codes P0131 for short to ground or P0132 for short to battery.
The hot test will be repeated every 20 seconds, as long as the circuit measurements are still out of range. This will continue until the key is switched off or until the measurements are in range.
The Cold test is performed at power down (ignition off) as long as the coolant temperature is at or below 185°F (85°C). This diagnostic will run after a generic short to battery or short to ground fault is set to identify the specific circuit which failed.