The oxygen sensor (O 2 ) is a galvanic battery that produces an electrical voltage when exposed to the oxygen present in the exhaust gases. Some oxygen sensors are electrically heated internally for faster switching when the engine is running. The oxygen sensor produces a voltage within 0 and 1 volt, providing this signal to the PCM that interprets it to learn how much oxygen is in the exhaust.
When there is a large amount of oxygen present (lean mixture), the sensor produces a low voltage (less than 0.4 volt). When there is a lesser amount present (rich mixture) it produces a higher voltage (0.6-1.0 volt). The stoichiometric or correct fuel to air ratio will read between 0.4 and 0.6 volt. By monitoring the oxygen content and converting it to electrical voltage, the sensor acts as a rich-lean switch. The PCM signals the power module to trigger the fuel injector and maintains the 14.7:1 air/fuel ratio necessary for proper engine operation and emissions control.
Two or four sensors may be fitted, depending on engine type and emissions package. Sensors are positioned before and after the catalytic converter and may be installed on both sides of the exhaust system. The one before the catalyst measures the exhaust emissions right out of the engine, and sends the signal to the PCM about the state of the mixture as previously discussed. The second sensor reports the difference in the emissions after the exhaust gases have gone through the catalyst. This sensor reports to the PCM the amount of emissions reduction the catalyst is performing.
Engines equipped with either a downstream sensor or a post-catalytic sensor will monitor catalytic converter efficiency. If efficiency is below emission standards, the MIL will be illuminated and a DTC will be set.
The oxygen sensor will not work until a predetermined temperature is reached. Until this time the PCM is running in what is known as "OPEN-LOOP" operation. Open-loop means that the PCM has not yet begun to correct the air-to-fuel ratio by reading the oxygen sensor. After the engine reaches operating temperature, the PCM will monitor the oxygen sensor and correct the air/fuel ratio from the sensor's readings. This is what is known as "CLOSED-LOOP" operation.
A heated oxygen sensor has a heating element that keeps the sensor at proper operating temperature during all operating modes. Maintaining correct sensor temperature at all times allows the system to enter into closed loop operation sooner.
The O 2 sensor uses a Positive Thermal Co-efficient (PTC) heater element. As temperature increases, resistance increases. At temperatures around 70-F, the resistance of the heating element is about 4.5 ohms. As the sensor's temperature increases, the resistance in the heater element increases. This allows the heater to maintain the optimum operating temperature of about 930--1100-F (500--600-C). Although the sensors operate the same, there are physical differences, due to the environment they operate in, that keep them from being interchangeable.
In Closed-loop operation, the PCM monitors the sensor input (along with other inputs) and adjusts the injector pulse width accordingly. During open-loop operation the PCM ignores the sensor input and adjusts the injector pulse to a preprogrammed value based on other inputs.
Four wires are used on each sensor: a 12-volt feed circuit for the sensor heating element, a ground circuit for the heater element, a low-noise sensor return circuit to the PCM, and an input circuit from the sensor back to the PCM to detect sensor operation.
Removal & Installation
- Disconnect the negative battery cable.
- Raise and support the vehicle.
- Disconnect the wiring harness from the oxygen sensor.
- Remove the sensor using the appropriate tool.
- Clean the threads of the mount to remove any old anti-seize compound.
- If the old sensor is to be reused, apply anti-seize compound to its threads. New sensors come with the compound already applied.
- Install and tighten the sensor to 22 ft. lbs. (30 Nm). Connect the wiring harness.
- Lower the vehicle.
- Connect the negative battery cable.
- To test the O 2 sensor in a static mode, disconnect the wiring connector and connect an ohmmeter across the sensor's two (white) wires. Resistance should be 4-7 ohms. If an open circuit is detected, replace the O 2 sensor.
- Start the engine and bring it up to operating temperature.
- Raise and support the vehicle.
- Backprobe the O 2 sensor between the O 2 sensor output wire and ground with a suitable high impedance voltmeter.
- The O 2 sensor should be rapidly switching between 0 and 1v. If working properly, it should be switching from a lean mixture (less than 0.4 volt) to a rich mixture (0.6-1.0 volt), and back. The average voltage should fall between 0.4-0.6 volt.
- If the sensor switches slowly, or is stuck in the middle of the range, the O 2 may be faulty.
- If the sensor is stuck rich or lean, it most likely indicates a problem with the engine; for example, a vacuum leak would cause the O 2 to read a lean mixture, and a malfunctioning fuel pressure regulator would cause a rich mixture.
- If the O 2 sensor is above or below the specified range (0-1.0 volt), a wiring or computer problem is most likely the cause.
- Lower the vehicle.
- Turn the engine off.