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Oxygen Sensor Service
For more than 25 years, oxygen sensors, more commonly known as O2 sensors, have been used to measure the oxygen content in the exhaust stream of gasoline powered automotive engines. This information is used to adjust the air/fuel ratio. Used in conjunction with catalytic converters, O2 sensors assist in minimizing emissions and maximizing fuel mileage. When an O2 sensor malfunctions, not only do emission levels increase and gas mileage decreases, engine drivability also suffers. Diagnosing problems with O2 sensors, therefore, is a valuable skill that will allow you to repair customer vehicles quickly and accurately.
O2 SENSOR BASICS
The O2 sensor is basically a rich-lean switch. Figure 1 shows a typical O2 sensor that is used in a HEMI® engine equipped Dodge Challenger (LC body). By monitoring the oxygen content in the exhaust stream, this sensor produces a voltage that is inversely proportional to the amount of oxygen in the exhaust. In other words, a higher voltage is produced when less oxygen is present; conversely, a lower voltage is produced when more oxygen is present.
Specifically, when there is a large amount of oxygen in the exhaust caused by a lean condition, misfire or exhaust leak, the O2 sensor produces a low voltage, below 450 mV. When the oxygen content is lower, caused by a rich condition, the O2 sensor produces a higher voltage, above 450 mV. This information is used to calculate the fuel injector pulse width.
The O2 sensor is a very temperature sensitive device with readings taken when it is below 570º being inaccurate. As a result, the engine controller stays in Open Loop mode. To counteract this problem, the O2 sensor is equipped with a heating element to heat the ceramic sensing element. The heating element brings the O2 sensor to operating temperature and allows the controller to go into Closed Loop operation as quickly as possible after the engine is started.
Due to the importance of the heating element in the O2 sensor, it is tested by checking its resistance by the controller almost immediately after the engine is started. The same O2 sensor heater return pin used to read the heater resistance is capable of detecting an open circuit, a shorted high or shorted low condition.
When O2 sensors were first introduced, only one sensor was used and it was located before the catalytic converter (commonly referred to as the upstream O2 sensor). Over time, a second O2 sensor was installed downstream of the converter. This second sensor is used to monitor the efficiency of the converter. It operates in the same manner as the upstream sensor, but the voltage signal (oxygen content) it generates is not used to adjust the air/fuel ratio; rather, this signal is compared to the upstream to determine the efficiency of the catalytic converter.
When the catalytic converter is operating at peak efficiency, there is a difference between the readings from the two O2 sensors. But as the catalytic converter deteriorates over time, the downstream reading (which, in theory, is measuring the oxygen content of clean or treated exhaust) begins to approach the upstream reading (dirty or untreated exhaust).
DIAGNOSING O2 SENSOR PROBLEMS
The O2 sensor can fail in one of three ways:
➊ Slow response rate. The response rate is the time required for the O2 sensor to switch from a lean to rich signal after it is exposed to a richer than optimum air/fuel ratio or vice versa. The O2 sensor must be able to quickly detect the change in the air/fuel ratio; as the sensor ages, the change can become more difficult to detect. The rate of change that the O2 sensor detects is called the big slope.
➋ Reduced output voltage. The voltage output range is 2.5 to 5.0 volt. A good O2 sensor can easily generate any voltage output in this range as it is exposed to different oxygen concentrations; a malfunctioning O2 sensor can have difficulty changing beyond the threshold value.
➌ Heater performance. Heater operation was explained earlier; this type of failure is usually the most common type of failure seen with O2 sensors.
Fortunately, onboard diagnostics can assist when determining the cause of an O2 sensor problem when one arises. A Diagnostic Trouble Code (DTC) can be set for a number of problems with the O2 sensor. These codes include the following common conditions for both the upstream and downstream O2 sensor:
Armed with a scan tool and the appropriate Diagnostic Service Manual, you should be able to pin down the specific O2 sensor problem. In addition, a visual inspection of the O2 sensor can also determine a problem.
Wiring problems, such as a bad connector, can lead to operational issues. If a problem is suspected, always check the connector and the condition of the wire lead to the sensor. Be sure the connector is properly fitted to the O2 sensor and that the wire lead is not frayed and contacting the engine (this would cause the O2 sensor to short to ground).
REPLACING O2 SENSORS
Replacing O2 sensors is usually straightforward. For the most part, the biggest obstacles are finding the particular O2 sensor to replace and working around a hot engine and exhaust pipes. Figure 1 illustrates the locations of the four O2 sensors used on the 6.1L HEMI® V8 engine.
1. Raise and support the vehicle.
2. Disconnect the wire connector from the O2 sensor wire connector.
3. Remove the O2 sensor with the appropriate oxygen sensor removal and installation tool.
4. Clean the threads in the exhaust pipe using the appropriate tap.
Note: When replacing an O2 sensor, the RAM memory in the PCM must be cleared. This is easily done by (1) disconnecting the C-1 connector, or (2) momentarily disconnecting the negative battery cable. Failure to do this task might cause drivability problems.
5. Install the O2 sensor; tighten to 30 ft.-lbs.
Note: The threads of new O2 sensors are coated with an antiseize compound to aid removal; do not apply additional compound to the threads.
6. Connect the wire connector to the O2 sensor wire connector.
7. Lower the vehicle.
On the Dodge Challenger, however, there is another obstacle and that is access to the upstream O2 sensor on the left bank of the engine. There is minimal room between the O2 sensor and the body of the vehicle. The key to removing the O2 sensor is to thread the sensor out of the exhaust pipe with one hand and use the other hand to twist the wire and connector around the sensor as it is removed. When installing the new O2 sensor, the same procedure using two hands must be used.