Rear Oxygen Sensor

By David Sturtz, April 15, 2008

The rear oxygen (O2) sensor is mounted in the exhaust pipe after or inside the catalytic converter. It constantly monitors the oxygen content in the exhaust gases and transmits that data to the engine control computer.

Symptoms of a Bad or Failing Rear Oxygen Sensor

  • Poor gas mileage
  • Check Engine Light may come on
  • Vehicle may fail an emissions test

Rear Oxygen Sensor Related Repair Advice

  • The rear oxygen sensor should be inspected for proper operation before the catalytic converter is replaced to correct a "failed convertor" fault code. In some cases, a failed rear oxygen sensor can cause an erroneous "failed convertor" fault code.
  • Oxygen sensors can fail if they are contaminated with oil, coolant, or silicone. If fluid contamination has caused an oxygen sensor failure, the source of the fluid must be resolved or the new sensor may be ruined.
  • Intake and exhaust leaks can trigger an oxygen sensor trouble code, but the oxygen sensor does not need to be repaired. Fixing the leak will commonly resolve the problem.
  • Most newer vehicles now use oxygen sensors that contain a heating element to help them warm to operating temperature faster. These heating elements often fail, requiring replacement of the sensor.

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