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How To Check Your O2 Sensor At Home

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. Too much or too little oxygen in the air/fuel ratio can cause engine performance issues, decrease gas mileage, trigger your check engine light, fail your emissions test and even damage your catalytic converter. 

A bad oxygen sensor that doesn’t correctly detect and correct skewed oxygen ratios can lead to several trips to the auto repair shop and thousands of dollars in service and repair fees. Below, learn how to check your O2 sensor at home. 

What Is An O2 sensor?

Your car’s emission system is designed to reduce the amount of dangerous gasses discharged into the environment, and your oxygen  sensor is a critical component of this system.

o2 sensor
Denso 234-4622 Oxygen Sensor.
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The O2 sensor is a simple electronic device on or near the exhaust manifold with a tip sensor that is inserted into the exhaust pipe. Vehicles also typically have an O2 sensor by the catalytic converter. Outside of automotive applications, O2 sensors are used by divers, marine biologists, beer brewers, and in the production of pharmaceuticals.

How Do O2 Sensors Work?

The measurements are sent in real-time to the engine control unit (ECU) that will adjust the amount of oxygen based on the oxygen sensor’s signal. If the air-fuel mixture has too much oxygen, the engine is burning lean. If the fuel mixture has too little oxygen, the engine is burning rich.

If the O2 sensor is not measuring the oxygen level accurately then the ECU cannot adjust fuel/oxygen levels accurately.

When working properly, 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 to 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: How Do You Test If An Oxygen Sensor Test Is Bad?

To check the health of your O2 sensor at home, you need a few different tools. Be sure to have:

  • A 10 mega-ohm voltmeter or multimeter
  • 1 scan tool to figure out which O2 sensor is failing
  • A floor jack if you are testing the O2 sensor by the catalytic converter

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 condition situation. Disconnect the hose from the positive crankcase ventilation (PVC) connector 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 signal’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 fails to respond 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.

DIY O2 Maintenance: The Bottom line

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

If you feel confident that the oxygen 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.

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