How To Check an O2 Sensor At Home

Every car built since the early 1980 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.

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) which 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 illuminated “check engine” light and one of the other signs may indicate that the o2 sensor has failed. One of the most prominent symptoms of a failing or dead sensor is a decrease in fuel efficiency. If an oxygen sensor fails, the fuel mixture will be rich, meaning all the oxygen will get used up in the cylinder.

Experiencing problems with a fuel mixture that is too lean or too rich indicates that you’re dealing with a faulty oxygen sensor since downstream or diagnostic sensors are responsible for monitoring the exhaust leaving the catalytic converter, and would not cause similar problems.

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.

O2 Sensor Test – What Can Cause Issues with Oxygen Sensors

Before we dive deeper into the topic of how to test an O2 sensor, we should identify the potential causes that can lead to problems with O2 sensors. At the very beginning, it’s crucial to understand that seeing the OBD2 code during diagnostics does not necessarily mean something wrong is going on.

Oxygen sensors may simply do their job and inform you about a fuel/air mixture that contains too little fuel. A lean fuel mixture would trigger an OBDII code, but it’s not an indicator that your oxygen sensor is faulty.

What can cause oxygen sensors problems are an internal contaminant, an electrical issue, or the age, high mileage, and bad condition of your vehicle.

One of the best practices is to conduct regular O2 sensor testing. Unheated oxygen sensors require frequent testing every 30,000 miles or so. Heated oxygen sensors can be inspected every 60,000 miles since, having their own source of heat, they can be placed further down the stream and therefore are less exposed to contaminants and at lower risk of experiencing malfunctions.

Heated oxygen sensors are in a more favorable position compared to unheated oxygen sensors, but, on the other hand, they are more prone to electrical issues. The faulty heater circuit often causes problems with an oxygen sensor and triggers the OBD2 codes.

There’s also the matter of simple material fatigue. Oxygen sensors are constantly exposed to adverse conditions like exhaust gasses and extreme temperatures, which inevitably will make them more prone to experiencing issues with time.

How to tell which O2 sensor is bad and may need replacing? We present a detailed guide in the next section of this article.

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 the 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.

How To Check an O2 Sensor with an Oscilloscope?

How to test an oxygen sensor using an oscilloscope? If you have one at your disposal, here’s what you want to do.

  1. Make sure the oscilloscope inputs are isolated from the mains electricity to perform the O2 sensor testing properly.

  2. Remember to perform oxygen sensor testing on the cold engine.

  3. Place the oscilloscope’s probes on the oxygen sensor cell lines in a way that won’t interfere with the engine’s work once it’s on.

  4. Start the vehicle’s engine.

  5. Observe the oxygen sensor outputs. The desired outcome is when the oxygen sensor outputs are low while the engine is warming up and then reach an average value signaling the proper fuel mixture. At first, you should see output voltage readings quickly fluctuating between 0.1 and 1.0 volts.

  6. You should also pay attention to the converter sensor outputs. Pre-catalytic outputs should display readings between the rich mixture and lean mixture. Post-catalytic output readings will be much more stable.

  7. The time it takes for the voltage to change from 0.1 V to 1.0 V (the lean to rich response time) should not be longer than 300 milliseconds. The same goes for when the voltage changes from 1.0 V to 0.1 V (rich to lean response time).

  8. If that is not the case or you’re getting no fluctuation in voltage reading, it’s a sign that your oxygen sensors need replacing.

DIY O2 Maintenance and O2 Sensor Testing: 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 the 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.

Related Reading: 

Oxygen Sensor Frequently Asked Questions

What is the most common oxygen sensor?

The most common oxygen sensors are narrowband oxygen sensors. In fact, the most common O2 sensor type would be the Wideband Zirconia sensor. This sensor has four wires: one output wire, one ground wire, and two heater wires. For diagnostic purposes, the most important of them is the signal wire.

What are the 3 types of oxygen sensors?

Apart from the zirconia oxygen sensors, the two most popular types that can be found in most cars are wideband O2 sensors and Titania 02 sensors.

What is the normal voltage range for most O2 sensors?

A properly working oxygen sensor will display the rapidly changing output voltage between 0.1 and 1.0 volts. The time between these fluctuations should amount to 300 milliseconds.

Are the rear oxygen sensors necessary?

Downstream sensors are necessary because they help you monitor the condition of the catalytic converter. Without them, you would get CEL (check engine light) or MIL (malfunction indicator light) signals, so removing them from your car is not recommended.

Do I need to replace all of the sensors at the same time?

The most sensible solution is to replace oxygen sensors in pairs. You can also expect that in cars produced after 1996, after replacing only one sensor, especially the front engine monitoring sensor, an ECU will set trouble codes for other sensors. The code will usually be displayed within two months from the repair.

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