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How to test O2 sensor with OBD2

The work of the O2 sensor is to monitor how much unburned oxygen is in the exhaust as the exhaust exits the car engine. Additionally, the O2 sensor notifies the computer if the fuel mixture is burning rich or lean. Therefore, if the oxygen sensor is bad, it can affect your combustion intervals, engine timing, and other key functions.

Besides, more than 50% of emission test failures are generated by malfunctioned O2 sensors. That’s why it’s crucial to determine whether your oxygen sensor is working correctly or not. If not, it will cost you a lot in the long run. For this reason, we’re going to show you how to test the 02 sensors using OBD diagnostic scanner to know if it’s working correctly or not.

But before we get to the procedure of how to test the oxygen sensor, it’s vital to define what’s an O2 sensor and the different types of O2 sensors on the market.

What is an O2 sensor?

An oxygen sensor is a device that’s located in the exhaust stream. As stated earlier, the function of the O2 sensor is to monitor the air/fuel ratio in real-time to know whether the fuel is rich or lean. In short, it means measuring the amount of oxygen in the gas or liquid passing through the exhaust manifold.

Note that many people misunderstand the measuring procedure. The O2 sensor doesn’t measure the concentration of oxygen. However, it measures the difference between oxygen in the exhaust gas and oxygen in the air.

Aside from checking the air/fuel ratio, the oxygen sensor also decides whether the catalytic converter functions correctly.

What are the different types of O2 sensors on the market?

There are various types of oxygen sensors on the market. These sensors differ in how they are made, but they still perform the same function. You will have to select three types: zirconia sensors, titania sensors, and wideband sensors.

  • Zirconia sensors

This is the most common type of O2 sensor on the market. It can either be heated or unheated.

Heated Zirconia sensors are among the most reliable O2 sensors on the market; just like its name, this sensor features a heating circuit. So, when the car starts, it becomes warm. To get more reliable results from this sensor, ensure it’s properly heated.

Unheated Zirconia sensors – even though this is one of the oldest sensors on the market, it’s not as efficient as the heated sensor. This is because it depends on the heat from the exhaust to warm up. So, if the exhaust doesn’t produce enough heat, the results won’t be very accurate.

  • Titania sensors

This sensor is built from an entirely different type of ceramic than the Zirconia sensor. Besides, the titania sensor works slightly differently from a conventional sensor. That’s why most car makers and models do not prefer it.

Unlike a Zirconia sensor that creates voltage, a titania sensor lowers or raises resistance depending on whether the fuel is rich or lean.  

  • Wideband sensors

These are a unique type of O2 sensors on the market. Most wideband sensors are found on new vehicle models. On top of that, this sensor is very accurate when it comes to getting a perfect fuel mixture for different driving conditions.

How to test O2 sensor using OBDII diagnostic scanner

Step 1: Connect the diagnostic OBD2 scan tool to your vehicle

Begin by connecting the OBD2 diagnostic to your car via the carport. If you can’t see where to link your code reader to the vehicle, confirm with the owner’s manual of your car.

Step 2: Turn on the car’s engine

Turning on your car’s engine allows the scanner to communicate with the vehicle’s onboard system. Make sure to turn only the ignition and not all the way to start the car. Depending on the scanner you’re using, you may be required to enter your vehicle’s identification number.

Step 3: Monitor the oxygen sensor voltage

After connecting the scan tool, check the oxygen sensor voltage with the engine running and when the engine is off. Additionally, it would help if you knew that the O2 sensor voltage would change when you drive, usually from zero to 1 volt. The variation in voltage may happen for about 10 minutes.

Nonetheless, it’s vital to note that new vehicle models with air/fuel ratio sensors come with different operations based on manufacturers. For instance, some models work with varying voltage and others with varying voltage.

Step 4: Take note of the variation of your O2 sensor voltage

This is a critical step when it comes to testing oxygen sensors. Take note of how fast the O2 sensor voltage alternates from less than 0.5V (low voltage) to over 0.5V (high voltage). Not that there will be toggles or variations that take place various times a second.

Hence, if your oxygen sensor voltage remains below 0.5V, this is an indication that there is a lean exhaust code set, while a voltage that is higher than 0.5V will mean that there is a rich exhaust code.

Step 5: Inspect the codes on the OBD2 scan tool

Next, check the codes on the automotive diagnostic OBD2 scan tool. With the scanner on, you will see at least two legends on the screen. When you take a check the codes, they will either say “active codes” or “pending codes”. Other codes that may show up include past, current, or potential codes.

If it’s a live code, it will keep the check engine light on. It also means that the issue has not been corrected and has to be tackled. On the contrary, a pending code means there’s a pending issue. It’s a sign that the operation of an emission control system failed once, and if it fails once more, the check engine light will turn on. If the CEL turns on, the pending code will become an active code.

Step 6: Diagnose the problem

Finally, you should interpret the codes and know what part of the O2 sensor has a problem. If you detect the codes, you should diagnose the issue based on the codes that show up on your OBD II screen.

O2 Sensor codes interpretation

Knowing how to interpret the O2 sensor codes is vital as it helps you pinpoint the exact issue. All you need to do is to use the DTC Look-up Library of your diagnostic scan tool or visit the OBD2 codes and look for the principles you’ve got on the “Search Box.” You will get to know the meaning, causes, symptoms, and the diagnostic steps of the codes you have.

Some common O2 sensor codes are P0130, P0131, P014C, P014F, P0150, P0151, P015B, P0171, P0174, and much more.

Signs of a bad O2 sensor

If you want to know if your O2 sensor is bad, look out for these signs. These are as follows:

  • The check engine light comes on.

One of the first signs to look out for is the check engine light. When you see the check engine light come on, one of the possible issues that your car might be facing is a bad O2 sensor. The check engine light usually comes on when the car has an issue.

  • Foul exhaust odor

Another notable sign of a failing oxygen sensor is a robust and unpleasant odor originating from the exhaust. Ensure to repair the O2 sensor in time to avoid the odor from spreading to the car’s interior.

  • Bad gas mileage

If you’ve noticed that your car has become less efficient when it comes to fuel consumption, it may mean that your O2 sensor is bad. When the oxygen sensor becomes less effective, the air/fuel mixture becomes rich. This means more fuel is used.

  • Decreased vehicle performance

Apart from bad gas mileage, a failing O2 sensor will reduce your vehicle’s performance. This is because it may lead to various car problems, such as weak acceleration, engine stalling, or even the engine might run rough.  


Having a fully functioning O2 sensor is very crucial if your car has to pass the emissions test. Testing the O2 sensor with an OBD scanner is super easy if you know the procedures to follow. With this guide, you should no longer worry about having a failing oxygen sensor as you can check it in due time before going for the emission testing.