(O2) Oxygen Sensors – Function, Failure Symptoms, How To Test

(O2) Oxygen Sensors - Function, Failure Symptoms, How To Test
(O2) Oxygen Sensors - Function, Failure Symptoms, How To Test

Every vehicle built since the early 1980’s, has one or more (O2) oxygen sensors, integrated into the exhaust system.

So, (O2) oxygen sensors, are used to confirm, that the proper air fuel ratio, is actually reaching the engine.

Consequently, measuring the (O2) oxygen level in the exhaust, provides a good gauge of what the fuel to oxygen ratio actually is. Above all, its job is to measure, the amount of unburnt (O2) oxygen exiting the engine.

So, (O2) oxygen sensors, are a very important part of your vehicle’s emission system. The emission system is designed, to reduce the amount of dangerous gasses, discharged into the environment.

(O2) Oxygen Sensors - Function, Failure Symptoms, Testing Procedures
(O2) Oxygen Sensors – Function, Failure Symptoms, Testing Procedures

The level of oxygen in the exhaust, is measured and sent to the engine control unit (ECU) for analysis.

So, if the fuel mixture has too much oxygen, the engine is burning lean. But, if the fuel mixture has too little oxygen, the engine is burning rich.

In either case, the (ECU) needs this information, in order to adjust the fuel/oxygen ratio. So, if your vehicle has a defective sensor, it will not run efficiently.

Symptoms Of Bad Or Failing (O2) Oxygen Sensors

Firstly, the sensor is not easy to get to or observe, because of its physical location. For that reason, there are several warning signs that will alert you, that there may be a problem.

Several of the most obvious warning signs that the sensor is failing include:

  • Reduced gas mileage
  • A bad smell like rotten eggs, coming from the exhaust
  • The check engine light comes on
  • You notice that your engine idles roughly
  • The vehicle is suddenly hard to start

Check Engine Light (CIL)

Usually, a check engine light, will provide the first indication that you, might have a bad sensor.

Often, the engine code will simply state that, there is a problem with the sensor. Sometimes, it may say that, there is a “heater circuit malfunction.”

So, (O2) sensors, can be very expensive to replace. However, the problem might not be with the sensor at all. Because, other factors may be involved, it is a good idea to test your sensor, before replacing it.

Testing Procedures (O2) Oxygen Sensors

So, oxygen sensors, can be tested both, on or off the vehicle. However, by taking the oxygen sensor off, you are able to visually inspect the sensor.

(O2) Oxygen Sensors - Function, Failure Symptoms, Testing Procedures
(O2) Oxygen Sensors – Function, Failure Symptoms, Testing Procedures

And, may provide a good indication that there is a problem. Furthermore, making the sensor a little easier to test.

Using A Digital Voltmeter

Above all, the results of this test, may require further analysis of other components:

  • Maybe there is a loose vacuum hose, that is forcing the (O2) sensor to read a high level of oxygen.
  • Maybe a loose connection to the (O2) oxygen sensor, is causing it to incorrectly read the exhaust gasses.
  • You just don’t know, until you measure the (O2) oxygen sensors operating characteristics.

Let’s look at diagnosing the problem, by measuring the (O2) oxygen sensors, operating characteristics:

First, identify the specific (O2) oxygen sensor, that you want to do the test on. Depending on the year of your vehicle, there could be up to 5 sensors in the exhaust system. Fortunately, the computer (DTC), will pinpoint the specific (O2) oxygen sensor, that needs to be tested.

Oxygen Sensor Fault Code List
Oxygen Sensor Fault Code List

So, using the (DTC), you can refer to your owner’s manual to locate the sensor. Your owner’s manual will also, identify the signal wire, as many (O2) oxygen sensors, have multiple wires connected to them.

You will need a 10-megohm impedance digital voltmeter, for testing the (O2) oxygen sensor.

Multimeter Testing
Multimeter Testing

You should set it to the millivolt (mV) DC scale.

Now, start the vehicle and let it run, until it reaches operating temperature. This may take up to 20 minutes:

  • Once you have reached operating temperature, turn off the engine.
  • Now connect the red probe to the sensors signal wire and the black probe to a good ground.
  • To perform the actual test, start the engine again and check the voltage readings.
  • The sensors voltage should fluctuate within the 100mV – 900mV (0.10V to 0.90V) range.
  • If it is within this range, the sensor is operating normally and you can stop testing.

In case it is not within range, there is either an engine problem or the (O2) oxygen sensor is bad. If it appears to be bad, continue with the next steps.

Test the (O2) oxygen sensor response, to a lean fuel consumption situation:

  • Disconnect the hose from the positive crankcase ventilation (PVC) valve, which is located on the valve cover.
  • This will allow more air into the engine, so the voltmeter should read close to 200mV (0.20V).
PCV Valve System (PCV) (Positive Crankcase Ventilation System)
PCV Valve System (PCV) (Positive Crankcase Ventilation System)

If the voltmeter does not respond, the sensor is not functioning properly.

Reconnect the (PVC) hose to test the sensors response, to a rich fuel consumption situation:

  • To do this, disconnect the plastic hose connection to the air cleaner assembly.
  • Then, block the hose connection opening with a rag, to reduce the amount of air going into the engine.
  • Check the voltmeter. It should read close to 800mV (0.08V) due to the reduction of oxygen entering the engine.

If the sensor does not respond this way, it is not functioning properly.

Reconnect the hose to the air cleaner:

  • If the sensor responded correctly to the lean and rich fuel tests, another component, could be causing the problem.
  • The potential issues, could be a vacuum leak, ignition system or something similar.

Obviously, if the sensor did not respond properly, then it is bad and will need to be replaced.

Used Oxygen Sensor (O2) Comparison
Used Oxygen Sensor (O2) Comparison

So, after all your testing, you should know whether the sensor is bad or if something else is the problem. If you feel confident that the sensor is bad, just replace it.


So, remember, addressing the problem sooner than later, may save you from more serious issues. Obviously, the logical choice, is to replace the failed sensor yourself. Finally, saving a bunch of money on costly repairs.

Thank You !