How To Test Your Car’s O2 Sensor And Why You Should

How To Test Your Car’s O2 Sensor And Why You Should

Your car’s O2 sensor is an important part of your car because it’s linked to its emission system.

This system should perform at its peak to decrease the number of toxic gasses that are released by the exhaust, and which contribute to the air pollution problem.

What does an O2 sensor do? This sensor measures how much-unburned oxygen is being released by the engine.

By knowing how much oxygen is in the exhaust, you’ll be informed of whether or not your car has a good fuel-to-oxygen ratio.

If there’s too much, or not enough, oxygen, the O2 sensor won’t work properly.

The car’s ECU tweaks how much fuel enters the system according to the oxygen level that the O2 sensor measures, so this sensor is really important to keep in good condition! 


How To Tell If Your Car’s O2 Sensor Is Faulty

If you suspect this important sensor could have a problem, you’ll need to be on the lookout for some common signs. These include:

  • Rotten-egg smell from the exhaust. 
  • The “check engine” light comes on. 
  • The engine idles roughly or you find it doesn’t start easily. 
  • The car’s gas mileage is reduced. 
  • The car is not as fuel efficient as it used to be. 
  • The engine stalls. 
  • There’s black smoke coming from the tailpipe, which signals too much fuel is being released from the exhaust. 

How To Test Your Car’s O2 Sensor

How To Test Your Car’s O2 Sensor

You want to test this sensor to be sure if it’s experiencing a problem. There are two ways in which you can do this.

  • Use an OBD2 scanner. If your “check engine” light is on, that could be the first sign that your car’s O2 sensor is faulty. When testing it with an OBD2 scanner, the error code might state that there’s a ‘heater circuit malfunction’ or direct you to the O2 sensor.
  • What if you don’t have an OBD2 scanner? You’ll have to get a voltmeter. These cost less than $20 and they come in handy to test your O2 sensor.

How to test the O2 sensor with a voltmeter: 

  1. Start the car and let it run for about 20 minutes so that it’s at its optimal temperature. 
  2. Then, turn it off and connect the red probe of the voltmeter to the O2 sensor’s wire signal wire. The black probe should be grounded on the chassis.
  3. Turn the engine back on. Check the voltage reading on the voltmeter. If the O2 sensor is healthy, its voltage will range from 0.10V to 0.90V. If it’s not within this range, you’ll have to move on to the next steps.
  4. The next step is to remove the hose from the PVC (positive crankcase ventilation) valve. You can find it on the top of the engine, on a valve cover. Doing this will increase airflow into the car’s engine. The voltmeter should have a reading that’s close to 0.20V. If not, the sensor is not working in the way it should. 
  5. Connect the PVC hose so you can check how the sensor responds to rich fuel consumption. Then, remove the plastic hose connection to the air cleaner assembly. Put a rag or cloth in the connection so that you can decrease how much air gets into the car’s engine. 
  6. Now consult the voltmeter again. You should have a reading of about 0.08V. If not, the sensor is faulty.
  7. Reconnect the air cleaner and hose. 


  • If the O2 sensor didn’t perform well in any of the above tests, it’s faulty and will need to be replaced.
  • You might have more than one O2 sensor in the car. These sensors can be found in your car’s exhaust system, and are usually located on the exhaust manifold or within close proximity to the engine. 

How Do O2 Sensors Work?

Earlier in this article, we touched on some important things to know about O2 sensors and how they work, but let’s go deeper into their function.

These sensors produce their own voltage when they reach a hot temperature, usually around 600 degrees Fahrenheit.

To be able to handle high temperatures, O2 sensors have a tip that gets connected to the exhaust manifold, and on this tip is a bulb.

The bulb is coated with a layer of platinum which works as an electrode. The inside of the bulb is vented via the sensor body to the outside atmosphere.

Now, when the bulb’s exterior comes into contact with hot gases inside the car’s exhaust, the difference in oxygen levels between the outside temperature and bulb will produce a voltage that flows throughout the bulb.

If the fuel-to-air ratio of the engine is lean (which means there’s not enough fuel present), this will cause a low voltage.

If, on the other hand, the fuel ratio is too high with too much fuel inside it, the voltage will be high

It’s worth noting that there’s an upstream and downstream oxygen sensor. The O2 sensor is known as a downstream oxygen sensor.

It basically functions by measuring the ratio of air and fuel that gets released from the catalytic converter. By doing this, it ensures that the converter can work properly. 

What Happens If You Don’t Replace Malfunctioning O2 Sensors?

What Happens If You Don’t Replace Malfunctioning O2 Sensors?

If your car’s O2 sensor is bad or failing, the computer in your vehicle won’t set the right air-fuel ratio for the engine.

This means that you’ll encounter a variety of problems, such as less fuel efficiency, more emissions, and damaged components in the car, such as the catalytic converter.

Since the bad O2 sensor will result in more emissions being produced by the car, this could also cause your vehicle to fail its emissions test

How Much Does It Cost To Replace Your O2 Sensor?

It’s worth knowing how much you can expect to pay for a new O2 sensor. You’re looking at an average cost of between $250 to $400.

This will depend on the model and make of your car, and it’s worth bearing in mind that the O2 sensor part itself costs around $130 to $250 while the labor costs will usually be between $120 and $150.

Although that might seem expensive, it’s much less costly than if you had to pay for other problems that can be caused by a faulty or malfunctioning O2 sensor.

For example, a bad O2 sensor could harm the engine timing.

To fix this, you will have to repair the timing belt, but this comes with high labor costs because so many parts need to be removed before the mechanic can gain access to the belt that needs to be repaired – you’re looking at between $200 and $900 just for the labor, while the timing belt itself will cost between $25 and $50. 

Should You Clean Your O2 Sensors?

Should You Clean Your O2 Sensors?

It’s always a good part of car maintenance to clean your O2 sensors. You can do this easily by removing them and then soaking them in gasoline overnight.

However, to remove them from your car’s exhaust system you will have to lift your car up on a jack.

If you battle to remove the O2 sensor from your car, you will find using a bit of WD-40 will be helpful to loosen them.

You should soak the sensors for at least eight hours and make sure that you cover the gasoline container with a sealed-tight lid so that fumes can’t escape.

When they have finished soaking, remove the sensors from the gasoline (remember to wear protective gloves!) and use a cotton cloth to wipe them dry.

Cleaning your O2 sensors regularly is important to prevent dirt from accumulating on them and making them malfunction. 

Related Questions 

How often should you replace an O2 sensor?


It’s usually recommended that you change your car’s O2 sensor every 100,000 miles. This can be costly, especially since some cars have up to four O2 sensors.

Do you have to remove the O2 sensor when testing it? 

You can leave the O2 sensor on the car when testing it, but removing it is a good idea because it enables you to inspect its wires and make sure that they’re connected properly.


If you want to be sure that your car’s O2 sensor is working properly, you should test it.

In this article, we’ve outlined how to go about testing your car’s O2 sensor so that you can tell if it needs to be replaced.

We’ve also looked at important information you need to know so you can ensure your O2 sensors continue working at their best, such as how to look after them by giving them a good clean every now and then.  


Leave a Comment

Your email address will not be published. Required fields are marked *