O2 Sensors

O2 sensors basically sniffs the exhaust and outputs voltage between 0 and 1 volt indicating how much oxygen is in the exhaust. The PCM reads the output voltage from the O2 sensor to adjust the rich/lean condition of the fuel mixture.

O2 Sensors Information

Where are the sensor/s located?

The oxygen sensor is located in the exhaust pipe just below where the pipe bolts to the exhaust manifold/header.

Will it trigger the check engine light if the sensor is faulty?

Yes it can and most of time does if it’s faulty. If it’s just the heater part going bad it may not light the DTC light. If the heater part is bad, it can cause poor running characteristics until the exhaust brings it up to the needed temperature. Now if the sniffing part of the sensor is bad it can/will cause poor performance, poor performance all the time. If it’s bad enough you will notice heavy black carbon build up around where the exhaust exits the pipe. It is even possible to see black smoke while driving from running too rich.

Why does it fail?

It is really hot where that little sensor lives. Some have internal heaters that can go bad from normal use. If your Jeep uses lots of oil and it’s getting burned going out the exhaust that can shorten the lifespan of the sensor. Oil or water leaks going out the exhaust. Basically anything that isn’t hot exhaust can shorten its lifespan. Some people include the 02 sensor as part of a tune-up procedure. If it’s not crazy expensive that would be a good idea. Save some gas mileage.

How do you test O2 sensors to see if they are bad?

To test the heater part all you need is a multi-meter.

With the sensor at room temp around 77 degrees F (25 degrees C). Put the both red and black lead across the white wires. Resistance should read between 5 and 7 ohms between the two wires. Replace the sensor if the meter reading shows 0 ohms or meter display doesn’t change at all.


Testing the function of the sensor you will need the multi-meter and a propane torch.

  1. First of all the “Check engine” light will likely give you a heads up on this one. Just run the DTC trouble codes and see what the PCM tells ya.
  2. To do the actual test to the sensor, clamp the sensor in a vise, vise grips or anything that will hold the sensor while you are testing it.
  3. Put the black lead to the body of the sensor and the red lead to the black sensor wire.
  4. Start heating the sensor; you should see the voltage start to climb until it gets to 1 volt.
  5. If you do not see a smooth steady progression to ~1 volt as the sensor heats up, replace the sensor.