O2 Sensor: What it is? How to Test a Bad O2 Sensor?

The O2 sensor is a critical component of your cars emission system. The emission system is designed to reduce the amount of dangerous gasses discharged into the environment.

Every car built since the early 1980’s has an O2 sensor integrated into the exhaust system. It was put there to measure the amount of unburned oxygen exiting the engine. Measurement of the oxygen level in the exhaust provides a good gauge of the fuel to oxygen ratio.

The level of oxygen measured is sent to the engine control unit (ECU) for analysis. If the fuel mixture has too much oxygen, the engine is burning lean. 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. If your car has a defective o2 sensor, it will not run efficiently.

So the point is how do you know if the oxygen sensor is bad? More importantly, how to test a bad O2 sensor?

What is an O2 sensor?

The O2 sensor (or Oxygen sensor) is physically located on the cars exhaust pipe. It is a simple device with a tip sensor that is inserted into the exhaust pipe. It is designed to measure the percentage of oxygen in the exhaust gasses.

o2 sensor
Denso 234-4622 Oxygen Sensor.

How does the O2 sensor work?

The measurements are sent in real-time to the ECU that will adjust the fuel/oxygen mixture as needed. If the O2 sensor is not measuring the oxygen level accurately then the ECU cannot adjust fuel/oxygen levels accurately.

The ECU will adjust the amount of fuel entering the system based on the oxygen level measured by the O2 sensor. It is important to remember that if the mixture of fuel and oxygen is incorrect, the amount of pollutants coming out of your car’s exhaust will increase. This not only harms the environment but could lead to damage of your catalytic converter or engine.

Symptoms of a Bad or Failing Oxygen Sensor

The oxygen 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 signs that the oxygen 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 car is suddenly hard to start

A combination of the check engine light and one of the other signs may indicate that the o2 sensor has failed.

The best way to know for sure is to identify the Diagnostic Trouble Code (DTC) that is stored in the ECU. If the code indicates that the o2 sensor is bad, you should perform additional testing to know for sure.

Oxygen Sensor Test

So how do you determine if the o2 sensor needs to be replaced? The answer to, how to test a bad o2 sensor, will ultimately be determined by your ability to perform diagnostics.

A voltmeter will provide you with the specific measurements, but the results 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. Or maybe a loose connection to the o2 sensor is causing it to incorrectly read the exhaust gasses. You just don’t know until you dig in and get your hands dirty by measuring the o2 sensors operating characteristics.

If you’re ready, let’s look at diagnosing the problem by measuring the o2 sensor’s operating characteristics, step by step:

  1. Identify the specific o2 sensor that you want to do the oxygen sensor test on. Depending on the year of your car there could be up to 5 o2 sensors located along the exhaust system. Fortunately, the computer DTC will pinpoint the specific o2 sensor that needs to be tested. 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 sensors have multiple wires connected to it.
  2. You will need a 10-megaohm impedance digital voltmeter for testing the o2 sensor. You should set it to the millivolt (mV) DC scale.
  3. Now start the car and let it run until it reaches operating temperature. This may take up to 20 minutes.
  4. Once you have reached operating temperature, turn off the engine. Now connect the red probe to the o2 sensor’s signal wire and the black probe to a good ground. Please use caution when connecting the probes as the engine and exhaust system will be extremely hot.
  5. To perform the actual test, start the car again and check the voltmeter’s voltage readings. The o2 sensor’s voltage should fluctuate within the 100mV – 900mV (0.10V to 0.90V) range. If it is within this range, the o2 sensor is operating normally and you can stop testing. In case it is not within range, there is either an engine problem (loose hose) or the o2 sensor is bad. If it appears to be bad, continue with the next steps.
  6. Test the o2 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). If the voltmeter does not respond, the o2 sensor is not functioning properly.
  7. Reconnect the PVC hose to test the o2 sensor’s response to a rich fuel consumption situation. To do this disconnect the plastic hose connection to the air cleaner assembly. Block the hose connection opening with a rag in order to reduce the amount of air going into the engine.
  8. Check the voltmeter. It should read close to 800mV (0.08V) due to the reduction of oxygen entering the engine. If the o2 sensor does not respond this way, it is not functioning properly.
  9. Reconnect the hose to the air cleaner.
  10. If the o2 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 o2 sensor did not respond properly then it is bad and will need to be replaced.

Bottom line

So, after all your testing you should know whether the o2 sensor is bad or if something else is the problem.

If you feel confident that the o2 sensor is bad, you can tackle it yourself. If you’re not sure that the o2 sensor is bad, you should probably take your car to a professional.

Remember, addressing the problem sooner than later may save you from more serious issues such as replacement of the catalytic converter. That part will cost you anywhere from $500 – $1000 to replace. Obviously, the logical choice is to replace the failed o2 sensor and save a bunch of money on costly repairs.

Tips: Use an OBD2 scanner to diagnose O2 sensor.

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