GM Century/Lumina/Grand Prix/Intrigue 1997-2000

Oxygen Sensors



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.

Click image to see an enlarged view

Fig. Locations of the heated oxygen sensors in the exhaust manifold and the three-way catalytic converter-3.4L (VIN E) engine shown, others similar

Catalyst Monitor Heated Oxygen Sensor (HO2S 2)

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.

Because of the proximity of the oxygen sensors and their wiring to the hot exhaust system components, a careful check should be made of the oxygen sensor wiring. The sensor pigtail may be routed incorrectly and contacting the exhaust system.
Check for poor PCM to engine block ground connections, especially on high-mileage vehicles or where there has been some major work such as engine removal and it is possible the ground connections were not properly installed.
If the fuel system pressure is too low, the system could go lean. While the PCM can compensate to some degree, if the fuel pressure is too low, an oxygen sensor DTC may set. A bad fuel injector could also cause an oxygen sensor related DTC.
Check for vacuum leaks. Look for disconnected or damaged vacuum hoses and for vacuum leaks at the intake manifold, throttle body, EGR system and crankcase ventilation system.
Check for exhaust leaks. An exhaust leak may cause outside air to be pulled into the exhaust gas stream past the oxygen sensor, causing the system to appear lean. Check for exhaust leaks that may cause a false lean condition to be indicated.
A faulty MAF sensor could cause a problem with the oxygen sensor system. A technician may disconnect the MAF sensor while watching his scan tool display. If, after disconnecting the MAF sensor, the lean condition is corrected, the MAF sensor likely needs to be replaced.
Fuel contamination could be a problem. Water, even in small amounts, can be delivered to the fuel injectors. The water can cause a leak exhaust to be indicated. Excessive alcohol in the fuel can also cause this condition.
Check the EVAP canister for fuel saturation, causing a rich mixture condition.
A leaking fuel pressure regulator can raise the fuel pressure, causing a rich mixture condition.
An intermittent TP sensor output could cause the system to go rich due to a false indication of the engine accelerating.
If, after removing an oxygen sensor, a white powdery deposit on the end of the sensor is found, it indicates silicon contamination, sometimes due to over-use of silicone sealers, or use of sealers in the wrong location. Replace the sensor.
Check for damaged or otherwise poor connections at the PCM. Inspect the harness for abrasion and/or heat damage.

Click image to see an enlarged view

Fig. The HO2S can be monitored with an appropriate and Data-stream capable scan tool

  1. Perform a visual inspection on the sensor as follows:
    1. Remove the sensor from the exhaust.
    3. If the sensor tip has a black/sooty deposit, this may indicate a rich fuel mixture.
    5. If the sensor tip has a white gritty deposit, this may indicate an internal anti-freeze leak.
    7. If the sensor tip has a brown deposit, this could indicate oil consumption.


All these contaminates can destroy the sensor, if the problem is not repaired the new sensor will also be damaged.


The oxygen sensors each use a permanently attached pigtail and connector. This pigtail should not be removed from the oxygen sensor. Damage or removal of the pigtail or connector could affect proper operation of the oxygen sensor.

Take care when handling an oxygen sensor. The in-line electrical connector and louvered end must be kept free of grease, dirt or other contaminants. Avoid using cleaning solvents of any type. DO NOT drop or roughly handle any oxygen sensor. GM says, "A dropped sensor is a bad sensor." A special anti-seize compound is used on the oxygen sensor threads. The compound consists of graphite suspended in fluid and glass beads. The graphite will burn away, but the glass beads will remain, hopefully making the sensor easier to remove. New or service sensors will already have the compound applied to the threads. If a sensor is removed from an engine and if for any reason is to be reinstalled, the threads must have anti-seize compound applied before reinstallation.

The heated oxygen sensor may be difficult to remove when the engine temperature is less than 120°F. Excessive force may damage the threads in the exhaust manifold or the exhaust pipe.

  1. Disconnect the negative battery cable.
  3. Raise and safely support the vehicle.
  5. If working on the HO2S 1, remove the intermediate exhaust pipe and the heat shield.

Click image to see an enlarged view

Fig. Note this special socket with a cutaway slot to accommodate the oxygen sensor's wire harness (pigtail)

  1. 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.

To install:

  1. Coat the threads of the oxygen sensor with a suitable anti-seize compound, if necessary.
  3. Install the sensor and torque to 30 ft. lbs. (41 Nm).
  5. Install the remainder of the components in the reverse order of removal.
  7. Connect the negative battery cable.