All of the models covered by this guide have two Oxygen 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 that are on your vehicle. Each type of sensor is describe below.Heated Oxygen Sensor (HO2S 1)
The fuel control Heated Oxygen Sensor (HO2S 1) is mounted in the exhaust manifold where it can monitor the oxygen content of the exhaust gas stream. The oxygen present in the exhaust gas reacts with the sensor to produce a voltage output. This voltage should constantly fluctuate from approximately 100mV (high oxygen content - lean mixture) to 900mV (low oxygen content - rich mixture). The heated oxygen sensor voltage can be monitored with a scan tool. By monitoring the voltage output of the oxygen sensor, the PCM calculates what fuel mixture command to give the injectors. For example, if the PCM reads a low HO2S voltage, it senses a lean mixture and commands more fuel. If the PCM reads a high HO2S voltage, it senses a rich mixture and commands less fuel.
To control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The PCM has the ability to monitor this process using the HO2S 1 and the HO2S 2 heated oxygen sensors. The HO2S 1 sensor produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way converter. The HO2S 2 sensor produces an output signal which indicates the oxygen storage capacity of the catalyst; this, in turn, indicates the catalyst's ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the HO2S 1 signal will be far more active than the signal produced by the HO2S 2 sensor.
The catalyst monitor sensors operate the same as the fuel control sensors. Although the HO2S 2 sensor's main function is catalyst monitoring, it also plays a limited role in fuel control. If the sensor output indicates a voltage either above or below the 450 millivolt bias voltage for an extended period of time, the PCM will make a sight adjustment to fuel trim to ensure that fuel delivery is correct for catalyst monitoring.
As with most all engine control sensors used in your vehicle, a thorough and proper test can only be performed by a qualified technician using a Scan Tool to read the data stream from the PCM. There are a few items a non-professional should check before taking the vehicle to a qualified technician for diagnosis and repair.
Perform a visual inspection on the sensor as follows:
- Remove the sensor from the exhaust.
- If the sensor tip has a black/sooty deposit, this may indicate a rich fuel mixture.
- If the sensor tip has a white gritty deposit, this may indicate an internal anti-freeze leak.
- If the sensor tip has a brown deposit, this could indicate oil consumption.
REMOVAL & INSTALLATION
- Disconnect the negative battery cable.
- Raise and safely support the vehicle.
- If working on the HO2S 1, remove the intermediate exhaust pipe and the heat shield.
- Detach the electrical connector and carefully remove the oxygen sensor. Because the permanently attached pigtail may be difficult to work around, note that there are special wrench sockets made with the side removed to accommodate the pigtail.
- Coat the threads of the oxygen sensor with a suitable anti-seize compound, if necessary.
- Install the sensor and torque to 30 ft. lbs. (41 Nm).
- Install the remainder of the components in the reverse order of removal.
- Connect the negative battery cable.