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How To Test O2 Sensor – Here’s What You Need To Know

How To Test O2 Sensor – Here’s What You Need To Know

There are certain ways you can tell that your o2 sensor is bad – you can figure this out by testing the o2 sensor in your vehicle. Oxygen sensors are fairly cheap when compared to other repairs and replacements in your vehicle, so changing them and testing them can help prevent any further issues in your car. The o2 sensor has a vital function of checking the car’s engine’s air and fuel mixture ratio. If the o2 sensor realizes that the mixture ratio is not correct, the car’s internal computer has the power to adjust the mixture. 

Auto Repairs Are EXPENSIVE


Without the o2 sensor working properly, the computer will not know how to adjust the air and fuel mixture. The amount of o2 in the engine depends on the other temperature in the car and the outside air, the altitude, the barometric pressure, the engine’s temperature, the engine load, and stress, and other factors. To keep the mixture at the correct ratio, you need to learn how to test the o2 sensor. 

What is the o2 sensor?


The o2 sensor, also known as the oxygen sensor, is located on your vehicle’s exhaust pipe. This device uses a tip sensor inserted into the exhaust pipe, designed to analyze and measure the percentage of oxygen within the exhaust gases, giving clues to the engine’s computer as to the health and ratio of the air and fuel mixture.

What is the function? 

The air and fuel mixture results within the engine are sent to the engine control unit of the car’s computer. This part will then change the air and fuel mixture as needed, either making the mixture more or less rich. If the oxygen sensor is broken and not measuring the o2 level accurately and quickly, the engine control unit will not be able to change the air and fuel mixture to the right levels.


The engine control unit changes the amount of fuel entering the system. If the o2 sensor is not working, the engine control unit will allow too much or too little fuel in the system, causing the pollutants and harmful gases in your car’s engine to increase. Learning how to test your o2 sensor can prevent damage to your car’s transmission, engine, or catalytic converter. 

Signs your o2 sensor is bad


There are common signs that your o2 sensor is bad, helping you to figure out why you need to learn how to test the o2 sensor. 


  • Glowing check engine light



Although your check engine light can turn on for a variety of reasons, some severe and some minor, the o2 sensor can cause this light to illuminate on your dashboard. Your check engine light might turn on due to the faulty mass airflow sensor, a loose fuel cap, or other more severe issues. Learning how to test your o2 sensor can prevent this check engine light turning on. Make sure you bring your car to a professional to find out the main root of the problem. 


  • Bad gas mileage



If you notice that your car cannot go as far for the same amount of gas, you have a problem with your fuel system and your fuel economy. If you are spending more money than usual on fuel and the price to fill up your tank is more than normal, the vehicle could have a damaged or poor o2 sensor. The engine will not be able to produce the same amount of power on the same amount of fuel as normal, meaning it is less efficient.


The lack of o2 efficiency here is due to the air and fuel mixture being too rich or too lean since the o2 sensor will not be able to analyze the mixture and alert the computer to changes. The o2 sensor becomes less efficient over time, so you might notice that the fuel economy will steadily get worse over time. If this is the case, you need to look at how to test your o2 sensor. 


  • Rough engine



Another sign that you need to learn how to test your o2 sensor is the rough engine idling or running rough. If your engine usually runs smooth and has a steady acceleration, then you need to figure out why it is suddenly running rough and jerking while accelerating. If the o2 sensor is not working correctly, it could produce loud noises during idle. The damaged o2 sensor can impact the engine’s timing, combustion timing, and other key functions in your car. 


  • Emissions test failure 



The emissions test is used to determine the health and the performance of your car, ensuring that the vapors and fumes being emitted from your car can pass the inspection to determine your vehicle is able to drive the road. Most emissions test failures come from a failed o2 sensor, meaning you need to learn how to test your o2 sensor. If you do not replace the bad sensor, you could end up spending much more on an expensive repair or replacement to keep your vehicle working at the optimal level. At this point in time, you might notice a bad smell along with the emission test failure, leading to you needing to learn how to test your o2 sensor. 


  • Old model vehicle



As you know, any mechanism or part in your car can break down over time, whether due to consistent damage or frequent wear and tear. Your o2 sensors are no exception, as they can become filled with contaminants, products of combustion, and oil ash that has built up over time. This debris build-up prevents the o2 sensor from sending the right signals to your engine’s computer about the air and fuel mixture. To solve this issue, you need to learn the steps to test the o2 sensor. 

Test Your O2 Sensor


To learn how to test your o2 sensor, you need to have certain tools and knowledge of the correct steps to take to troubleshoot your issues. To run the test of your o2 sensor, you need to connect the impedance digital voltmeter. Before you connect the voltmeter, locate the o2 sensor near the exhaust manifold or the catalytic converter.


When you find the diagnostic trouble codes that are stored within the system to give you an idea of the direct problems, you might also learn some important info about the specific o2 sensor that is damaged or at fault. If you get a certain trouble code related to a specific part, you can narrow down the issues to a certain system.


If you need to find a Bank I and Bank II code, this can alert you to your catalytic converter or cylinder head problems. Locate the signal wire within the system to determine the root cause of the problems and how to test the o2 sensor. 


  • Warm Up Your Engine



Once you have the voltmeter and found the o2 sensor in your car, you need to warm up your car to get the proper operating temperature level. You can do this by driving your car around for a certain amount of time, usually between 15-20 minutes, to get your car to the proper speed and temperature. 


  • Turn Off The Engine


Once the engine has warmed up over time, you need to set the voltmeter to the specific setting – the DC scale.


  • Lift The Vehicle


To access enough of the car and gain access to test the o2 sensor, you need to lift your vehicle using a jack. Support the vehicle on jack stands and block the ends of the vehicle so it does not move. This step is completely necessary if you are testing the o2 sensor using the catalytic converter access point. 


  • Connect The Meter



Be careful when you connect the voltmeter to the o2 sensor, as the engine will have warmed up to the proper operating temperature, which can be extremely hot. Make sure to not touch any extraneous parts, wince the exhaust manifold and internal pipes are very hot. Keep the probes away from hot surfaces to prevent friction, burning, or melting. 


  • Connect The Probes To The Sensors



If your o2 sensor has more than one wire, you will have to connect the red probe to the sensor wire and the black probe to the ground wire on the car’s engine. If your car has multiple wires, like more than four wires, connect the voltmeter’s black probe to the ground wire. This is a common way of how to test your o2 sensor. 

Reading the O2 Sensor Signals


To learn how to test the o2 sensor, you need to learn how to analyze and decipher the oxygen sensor signals. First, you need to turn on the engine and check the voltage signals coming from the voltmeter. The sensor should go between a certain range to show it is working properly, usually fluctuating between 100 mV and 900 mV. To check the sensor operating correctly, you have two tests to run. 


  • Test Response To Lean Fuel Condition



In this situation, the first step is to remove the hose from the positive crankcase \ventilation valve. After the hose has been disconnected, more air will enter the engine at one time. If you need to find the correct ventilation valve, check the manual for your specific car or visit your local mechanic.


The next step is to check the signal voltmeter reading. The o2 sensor analyzes the increase in oxygen with the air and fuel mixture. If it notices the mixture has become too lean due to the excess oxygen, the sensor will produce a signal that is close to the 200 mV mark. You can tell the sensor is not working correctly if the sensor takes too long to respond to the fuel mixture. The last step is to reconnect the hose to the PCV valve.


  • Test Response To Rich Fuel Condition



When learning how to test the o2 sensor, you need to learn how to test the o2 sensor response to your vehicle’s rich fuel condition. The first step is to remove the plastic duct from the air cleaner in your car. Cover the duct opening with a rag to reduce the amount of air entering the engine. 


Next, check the o2 sensor’s voltmeter reading, which should be close to around 800 mV. If the sensor does not respond at the right time or the right away, the sensor will not be working correctly, and you need to get it repaired or replaced. The final step is to reconnect the air duct to the cleaner and turn off the entire car. 

O2 Sensor Replacement Cost


A faulty sensor can cause a variety of issues in your vehicle, like poor fuel consumption and potentially detrimental and non-reversible engine damage, leading to an expensive replacement. However, if you choose to replace the o2 sensor before it causes further damage, it will be much less costly in the long run.


The cost to replace the o2 sensor on your own if you have some mechanical knowledge runs between $20 and $94 on average. The price usually ranges between $113 and $478 for both the parts’ cost and the labor price at a shop. To get an idea of how much you might spend, we have included a replacement o2 sensor cost comparison of various places. 

O2 Sensor Cost Comparison


If you are bringing your car to your local mechanic, you can expect the o2 sensor replacement cost of range between $113 and $478. At a chain store, like Midas, the total cost runs between $145 and $398, while the Mr. Tire stores range between $113 and $478, the same price as your local mechanic. The other two chain stores you could bring your vehicle to is NAPA, costing you between $186 and $420, and the Pep Boys, charging you just between $50 and $70 for the parts only. For other parts-only stops, Walmart costs between $27 and $40 for the o2 sensor, while Amazon charges between $20 and $94 for the parts’ cost. 



As you can see, the o2 sensor is a key part of your fuel system in your vehicle. By keeping this part running smoothly, by knowing the signs and symptoms of a faulty o2 sensor and what causes them, you can keep your car healthy for a long period of time. Learning how to test your o2 sensor is crucial to the longevity and performance of your vehicle.