The oxygen (O2) sensor is a device which produces an electrical voltage when exposed to the oxygen present in the exhaust gases. The sensor is mounted in the exhaust system, usually in the manifold or a boss located on the down pipe before the catalyst.
Some models have two sensors, one before the catalyst and one after. This is done for a catalyst efficiency monitor that is a part of the OBD-II engine controls. 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 talked about. 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.
The oxygen sensor will not work until a predetermined temperature is reached, until this time the PCM is running in what as 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 comes to 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 (HO2S) 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.
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.
The HO2S detects the presence of oxygen in the exhaust and produces a variable voltage according to the amount of oxygen detected. A high concentration of oxygen (lean air/fuel ratio) in the exhaust produces a voltage signal less than 0.4 volt. A low concentration of oxygen (rich air/fuel ratio) produces a voltage signal greater than 0.6 volt. The HO2S provides feedback to the PCM indicating air/fuel ratio in order to achieve a near stoichiometric air/fuel ratio of 14.7:1 during closed loop engine operation. The HO2S generates a voltage between 0.0 and 1.1 volts.
Removal & Installation
- With vehicle in NEUTRAL, position it on a hoist.
- Disconnect the negative battery cable.
Disconnect the heated oxygen sensor (HO2S) electrical connector.
NOTEIf necessary, lubricate the HO2S with lock lubricant to aid in removal.
Remove the HO2S from the exhaust manifold using an oxygen senor wrench.
NOTENever reuse an oxygen senor when removed, always install a new sensor.
- Installation is the reverse of removal, tighten the sensor to 35 ft. lbs. (48 Nm).
- Disconnect the HO2S.
- Measure the resistance between PWR and GND terminals of the sensor. Resistance should be approximately 6 ohms at 68°F (20°C). If resistance is not within specification, the sensor's heater element is faulty.
With the HO2S connected and engine running, measure the voltage with a Digital Volt-Ohmmeter (DVOM) between terminals HO2S and SIG RTN (GND) of the oxygen sensor connector. Voltage should fluctuate between 0.01-1.0 volts. If voltage fluctuation is slow or voltage is not within specification, the
- Check the condition of the connector. Make sure the connector is firmly attached. Check for broken or bent connector pins. Repair any connector damage before continuing with troubleshooting the issue.
- Check the condition of the wiring to the connector. If the wiring is damaged, repair the wiring before continuing with any further tests.
- The battery should be fully charged and the starting system should be functioning properly.
Check for leaks at the following:
Hoses connecting to the mass air flow (MAF) sensor assembly
- Measure the resistance between: the HO2S positive terminal connector on the component side and the HO2S negative terminal connector on the component side. If the resistance is not between 3-30 ohms, replace the sensor.
- Measure the resistance between the positive terminal of the PCM connector on the harness side and the negative terminal of the HO2S connector on the harness side. If the resistance is not less than 5ohms, repair the open circuit in the wiring harness.