Car encyclopedia

Rear Oxygen Sensor Replacement

Rear oxygen sensors are key components of the vehicle's exhaust system and monitor the efficiency of the catalytic converter.

Over time, oxygen sensors become "lazy," and their output signal deteriorate. An engine that burns oil or has a blown head gasket (which allows antifreeze into the exhaust system) may contribute to the failure of the sensor. Modern sensors have an electrical heating circuit built into them, which is prone to failure over time.

To replace an oxygen sensor, its electrical connection is disconnected, the sensor is removed from the exhaust system, and the new sensor is installed

Mechanics Corner: More Technical Detail

Oxygen sensors constantly monitor the oxygen content in the exhaust gases and sends this information to the engine management computer 10 or more times per second.

The first oxygen sensors were mounted only in front of the catalytic converter, but as of 1996, an oxygen sensor is mounted before and after each catalytic converter.

  • The purpose of the front oxygen sensor, which is mounted before the catalytic converter, is to measure how rich or lean the gases are as the gases exit the combustion chambers. Depending upon whether the exhaust gas is lean (high in oxygen content) or rich (low in oxygen content), the amount of fuel entering the engine is adjusted by the engine management computer to try and maintain an ideal mixture that produces the lowest emissions output from the catalytic converter.
  • The purpose of the rear oxygen sensor is to monitor the oxygen content of the exhaust gases leaving the catalytic converter. This gives the emission system a way to monitor the operation and efficiency of the catalytic converters. The rear oxygen sensor checks several things in the post–catalytic converter gases. It measures the amount of oxygen, or peak threshold, in the exhaust gases and follows how fast the amount of oxygen changes. On some vehicles, the rear oxygen sensor is mounted midway in the catalytic converter to measure how efficiently the reduction bed convertes NOx by checking the amount of oxygen that is released back into the exhaust flow. 

With each new model year, manufacturers are adding more oxygen sensors to better manage engine operation. Some high-performance engines have an oxygen sensor for each cylinder as well as one for the rear of each catalytic converter: for an 8-cylinder engine, that can mean 10 oxygen sensors.

Replacing the front oxygen sensor varies from vehicle to vehicle. Some vehicles have only one front oxygen sensor and are as easy to replace as a single spark plug. Other vehicles have one or more oxygen sensors per bank, or group, of cylinders, and can be very difficult to access. The front oxygen sensor is usually on or near the exhaust manifold but in front of the catalytic converters. Usually, the electrical connector is first disconnected and then the sensor is unscrewed from the exhaust system. These initial steps can be rather tricky because some vehicles have oxygen sensor wiring that enters the passenger compartment, so  disconnecting and removing then can take half an hour or more to complete.

The performance of any oxygen sensor deteriorates over time: the amount of the signal change, or bandwidth, as well as the speed, or frequency, of signal change decreass. The engine management computer is constantly monitoring this performance, and when the programmed envelope of acceptable oxygen sensor performance has been breached, the computer will set a fault code and illuminate the check engine light.

  • In some cases, the oxygen sensor is contaminated by something in the exhaust gases, such as coolant from a leaking head gasket or oil from a worn engine. Even the wrong type of gasket sealer with silicone can contaminate, or foul, an oxygen sensor. If the sensor has any noticeable coloring from a leaking substance, the source of the substance must be located and repaired or any new oxygen sensor will be ruined in short order.
  • Another common reason for oxygen sensor failure is failure of the heating element. Most oxygen sensors have a heating element to rapidly bring the sensor up to the high (400 to 600 degrees Fahrenheit) operating temperature it needs. This heating element is a bit like the element in a hair dryer that glows when it is hot. Over time, the punishment dealt by the exhaust gases to the oxygen sensor will damage the heating element. This condition will set a check engine code that necessitates the replacement of the oxygen sensor.
  • There can be oxygen sensor problems that aren’t related to a faulty sensor. If there is an exhaust system leak that allows false, or unaccounted for, outside air to reach the sensor, it will produce false readings that are sent back to the engine computer. This may cause a false compensation to the air/fuel mixture that will produce dirtier vehicle emissions.
  • An engine misfire can pulse excessive amounts of oxygen to the oxygen sensor and set an oxygen sensor code when there isn’t a problem with the sensor itself.

Always have a trained emissions diagnostic technician inspect and determine why the oxygen sensor needs attention so that only the proper repair is performed. It is recommended to always use a factory-quality oxygen sensor with the proper connector. The use of a universal oxygen sensor can cause poor emissions system performance. This is due to the fact that since 1990, the operating tolerance allowed by engine computer programming gets more rigorous each model year. Universal oxygen sensors have thus become outmoded because their operating tolerance is too generic.

Ask a Question Find a Shop
Featured Articles
Ask a Question