Although the oxygen sensor is termed a sensor in actuality it is a galvanic battery. The oxygen sensor compares the potential difference between the ambient oxygen content around the exhaust and the oxygen content present in the exhaust stream. When the exhaust sample is lean there is more oxygen in the exhaust as compared to the atmosphere. When the exhaust sample is rich there is less oxygen content in the exhaust as compared to the atmosphere. The greater the difference between ambient oxygen and exhaust oxygen content the greater the voltage produced.
For the oxygen sensor(s) to operate properly it has to reach an operating temperature of approximately 600°F before a consistent voltage potential can be generated.
The Engine Management System (EMS) determines the state of readiness of the oxygen sensor(s) by supplying a bias voltage of approximately 400 - 500mVDC to the oxygen sensor. As the sensor begins to warm up the voltage produced increases due to rich exhaust mixtures commanded by the EMS. When the EMS senses a return voltage greater than the bias voltage the computer will command the fuel mixture lean. When the output voltage from the sensor drops below bias voltage levels the computer will command a rich mixture again. When the EMS determines that the O2 sensor has responded properly and within a predetermined amount of time it will begin using the sensor as an input to adjust the air/fuel mixture.
Typically oxygen sensors used in OBD II engine management systems incorporate heaters. These heaters raise the sensors up to operating temperature quickly. The sooner the oxygen sensor gets to operating temperature the sooner the EMS can maintain closer control over emissions, economy and performance. The oxygen sensor along with other sensors provides the computer with necessary information to maintain favorable operating conditions for the catalytic converter. The role of the catalytic converter is to store oxygen for the reduction of HC, CO and NOX emissions. The oxygen sensor input is used by the EMS to protect the catalytic converter by cycling the air/fuel mixture rich and lean. This provides adequate oxygen for storage while maintaining cool enough operating temperatures to prevent catalyst damage.
In addition to controlling the catalytic converter's operating conditions for emissions control the computer uses oxygen sensor data to tailor air/fuel mixture, providing a balance between fuel economy and performance.
Abnormal sensor activity has a profound effect on pulse-width and air/fuel mixture strategies. Sensor values that indicate lean conditions will cause the computer to command changes in short term fuel strategies. Conditions such as secondary misfires create excessive HC levels. This also produces high oxygen levels in the exhaust. The oxygen sensor will "sense" only the increased oxygen content and input to the computer will be below bias voltage levels. The computer will respond by commanding additional fuel.
OBD II vehicles use oxygen sensors downstream of the catalytic converter(s) to monitor the efficiency of the catalyst. When the catalyst performs properly available oxygen is used resulting in low oxygen levels in the exhaust sample after the catalyst. While downstream oxygen sensors are constructed the same as upstream oxygen sensors the values that they generate are different. With relatively richer mixtures present around the downstream oxygen sensor voltage inputs to the computer will be above the 450mV bias voltage. If the catalyst is operating effectively the downstream oxygen sensor will cycle when the catalyst is flooded with oxygen. Typical values from the downstream oxygen sensor(s) are between 550- 900mV at idle.
While the downstream oxygen sensor is used to monitor catalyst efficiency the upstream sensor has a pronounced effect on engine performance. Lean oxygen sensor values will result in an increased pulse-width, excessive emissions, surging, hesitation, and catalyst damage. Additional fuel can cause the catalyst temperatures to rise due to an afterburner effect in the converter. The oxygen sensor is the only post-combustion input to the EMS. Other malfunctioning systems also affect its operation.
An improper rich condition will cause the oxygen sensor to lean operating conditions. This may result in loss of power, hesitation, surging, poor idle quality and possibly converter damage. Sensors that do not switch properly, or are lazy, do not provide accurate information to allow the computer to properly maintain the air/fuel mixture. Faulty heaters do not allow the sensors to reach operating temperature fast enough and the vehicle may remain in open loop for longer periods of time. Malfunctioning heaters also allow the sensors to cool down during periods of extended idle.
Related Symptoms
A faulty oxygen sensor due to loose connections, bad grounds, high resistance in the circuit, or opens in the circuit can cause the following symptoms.
CAUTION
The oxygen sensor uses a permanently attached pigtail and connector. Do not remove the pigtail from the oxygen sensor. Damage to or removal of the pigtail connector could affect proper operation of the oxygen sensor.
CAUTION
The oxygen sensor in-line connector and louvered end must be kept clear of grease, dirt or other contaminants. Avoid using cleaning solvents of any type. DO NOT drop or roughly handle the oxygen sensor.
Note: Removal of the oxygen sensor is easier when the engine temperature is above 48°C (120°F).