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Do I Have a Bad Oxygen Sensor? Signs, Causes, and Solutions!

Do I Have a Bad Oxygen Sensor? Signs, Causes, and Solutions!

Do I have a bad oxygen sensor? If you have asked this question at some point, it can mean that your car is experiencing symptoms of a bad oxygen sensor which include engine overheating, poor fuel economy, check engine light, poor engine performance, rough idling, engine misfiring, stalling, and failed emission tests. Bad oxygen sensors are usually caused by numerous contaminants reaching into the exhaust or when it is near the end of its lifespan. To fix the problem, you will have to replace your bad oxygen sensor. However, if the problem that causes the contamination has not been corrected, such as a cracked cylinder wall, bad head gasket, or worn rings, the new oxygen sensor will most likely fail again. To know more about oxygen sensors, read on! We’ll go through the causes, solutions, and signs of a bad oxygen sensor!

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Do I Have a Bad Oxygen Sensor? – What is an Oxygen Sensor?


Modern cars are equipped with various sensors and each of them performs a specific task. These sensors are used to manage or regulate the temperature, oil pressure, coolant levels, emission levels, and more. They help make sure that your car is always at its optimum performance and is always fuel efficient. One of these important sensors is the oxygen sensor.


The oxygen sensor or the O2 sensor can be found within the emissions control system. It is designed and built to monitor the amount of oxygen in the exhaust stream of your car. It also confirms that the catalytic converter in your system is working properly and makes sure that the engine is operating at top performance.


The oxygen sensor monitors your emissions. It can detect whether your engine is operating an air and fuel mixture that is too lean or too rich by oxidizing a part of the oxygen as it enters through the exhaust system. This reading will then be converted into a voltage signal and will be sent to the car’s onboard computer or ECU.


The ECU will then alter the fuel metering and timing to keep the ideal mixture. This process is constantly changing in real time. The system will adjust according to the needs of the engine depending on the engine load, engine temperature, acceleration, warm up period, and other contributing factors. So when the ECU gets a too rich reading from the oxygen sensor, it adjusts the fuel mixture to lean to reduce the sensor’s voltage signal and when the sensor sends a too lean reading, the ECU will adjust the mixture again to make it go rich.


When you ask, do I have a bad oxygen sensor? Bear in mind that modern vehicles usually have more than one oxygen sensor, but V6 and V8 vehicles with dual exhausts usually have four oxygen sensors. In most cars, there is usually one upstream sensor and one downstream sensor of the catalytic converter. The upstream one is located closer to the engine which is on the exhaust pipe and the downstream sensor is found closer to the muffler. There are two oxygen sensors since they aim to adjust the input the ECU gets by getting two readings.


Do I Have a Bad Oxygen Sensor? – What Causes an Oxygen Sensor to Fail?



Do I have a bad oxygen sensor? You have probably asked this question when you suspect that your oxygen sensor has failed. But what causes an oxygen sensor to fail? The oxygen sensor is considered as one of the most frequently replaced car sensors. It is a very important sensor since it is the one responsible for maintaining a good fuel economy and low emissions. When the oxygen sensor becomes bad, the car’s onboard computer won’t be able to adjust the fuel mixture fast enough, as soon as the operating conditions of the engine change.


Now back to the question, what causes an oxygen sensor to fail? The oxygen sensor becomes faulty due to different kinds of contaminants that get into the exhaust. Problems such as internal coolant leaks due to a cracked cylinder wall or a leaking head gasket can cause silicates to contaminate the exhaust which can lead to an oxygen sensor failure. Worn rings or bad valve guides can also lead to excessive oil consumption which can contaminate the exhaust with phosphorus and damage the oxygen sensors. When this happens, an oxygen sensor replacement will be needed. However, if you fail to correct the problems that caused the exhaust contamination, it is most likely that the new oxygen sensor will fail again.


A contaminated fuel can also cause your oxygen sensor to go bad as well as external contamination such as undercoating materials, chemicals, and road salt. Since the oxygen sensors are also exposed to extreme temperatures, it will eventually become worn out. When this happens, it can cause a number of problems which can lead you to ask, “do I have a bad oxygen sensor?”.


Do I Have a Bad Oxygen Sensor? – Can You Visually Tell if an O2 Sensor is Bad?


To answer the question, “do I have a bad oxygen sensor?”, you can test your oxygen sensor to know whether it is bad or not. To do the test, you can choose between leaving it attached to your car or removing it off from your car to have it tested. You will need two tools to do an oxygen sensor test. You will need a back probe and a high-impedance digital voltmeter. To do the test, just follow these simple steps.


  • You need to identify which oxygen sensor you want to test. It can be confusing to know which is the faulty one since cars are usually equipped with more than one sensor. The number of oxygen sensors installed in your car can vary, depending on its year, make, and model. But the good news is, you can determine which one of the oxygen sensors needs to be tested through the DTC or the diagnostic trouble code. By looking at the stored DTC, you can consult your owner’s manual to find the location of the faulty oxygen sensor. You also need to check your owner’s manual to determine the signal wire since there are many oxygen sensors that have various wires connected to it.


  • When you know the location of the sensors, you can visually check it and its surrounding wires. You can look for any signs of damage or wear and tear. You also need to check if the wires are still intact.


  • Next step is to get your high impedance digital voltmeter. You need to set it to the mV or millivolt DC scale.


  • You can then start your car and let it run until its engine reaches its normal operating temperature. It will usually take around 20 minutes to achieve the desired engine temperature. When the engine’s normal operating temperature is achieved, turn off the engine.


  • Connect the red probe to the signal wire of the oxygen sensor and the black probe to a good ground. Make sure that you are wearing safety gloves and connect the probes carefully since the engine and exhaust system will be very hot.


  • Start your car again to do the actual test. Check the voltage readings on the voltmeter. To know that your oxygen sensor is working properly, the voltage of the sensor should be fluctuating within 100mV to 900mV or 0.10 to 0.90V range. If you have this reading, you can stop doing the test, but if it is not, then it means that you have a bad oxygen sensor. You can continue to do the next steps.


  • You need to test the oxygen sensor’s response to a lean fuel consumption state. To do this, find the positive crankcase or PCV valve hose and disconnect it. You will usually find it on the valve cover. Disconnecting the hose enables more air into the engine. Your voltmeter should read close to 200mV or 0.20V, but if the voltmeter doesn’t react, it means that the sensor is faulty.


  • Next is to test the oxygen sensor’s response to a rich fuel consumption state. This is done by reconnecting the PCV hose and by disconnecting the plastic hose connection to the air cleaner assembly. Then block the opening of the hose connection with a cloth to decrease the amount of air entering the engine.


  • Once done, check your voltmeter. It should have a reading of about 800V or 0.8v since the oxygen going into the engine is reduced. If it has a different voltage reading, then it means that it is faulty. You can then connect the hose back to the air cleaner assembly.



Do I Have a Bad Oxygen Sensor? –  What Does an O2 Sensor Do When it Goes Bad?



Do I have a bad oxygen sensor? If you experience any of the symptoms of a bad oxygen sensor, then you probably have a bad sensor. Its symptoms include:


  • Increased fuel consumption


If your oxygen sensor becomes bad, you will notice an increased fuel consumption. According to the EPA, an oxygen sensor failure can cause your fuel economy to drop by up to 40 percent. This can happen since a bad oxygen sensor negatively affects the fuel combustion and delivery systems of a car. It enables too much fuel to be injected into the engine which translates to higher fuel consumption.


  • Check engine light


Although the check engine light can be set off due to various reasons, a bad oxygen sensor is one of them. To determine what caused the check engine light to come on, you can check the fault code stored in your system.


  • Rough idling


Rough idling is one of the first symptoms of a failing oxygen sensor. It can happen since a failing sensor can affect the combustion mixture. The engine may run too rich or too lean, meaning there is too much air or too much fuel in the air to fuel ratio. When this happens, it can result in improper combustion which can lead to engine hesitation or rough idling.


  • Poor engine performance


If your car experiences rough idling, other problems may follow. When the oxygen and fuel ratio is thrown off, it will have a negative impact on your engine. It may struggle to operate at its optimal performance and you may notice that your car feels sluggish while you are driving.


A bad oxygen sensor can cause poor engine performance which includes issues with maintaining a constant speed, acceleration, and hesitation. These problems can also be caused by issues with other car components. It is best to have your car checked to determine what caused your engine to run poorly.


  • Engine overheating


Engine overheating is probably one of the less common problems that can happen when you have a bad oxygen sensor. Although it is not that common, it can still happen. This is due to the fact that too much fuel is constantly used for combustion. Your engine will continue to overheat until the faulty oxygen sensor is replaced since the oxygen sensor is the main regulator of the air and fuel mixture. Although there are other components in the car that can help prevent overheating, it will just be for a while. The issue should be addressed immediately since repeated overheating will eventually cause further damage.


  • Engine misfiring


Since a bad oxygen sensor can lead to a too lean or too rich air and fuel mixture, it can cause the explosions within the cylinders of the engine to be altered. When this happens, it can result in engine misfires, more so when the engine is idling.



If the answer to the question, “do I have a bad oxygen sensor?”, is yes, you will have to replace your faulty oxygen sensor. When buying replacement sensors, you need to get the same type as the original one. Make sure that you purchase one with the same number of wires. If your car has multiple oxygen sensors and one of them becomes bad, it is most likely that the other oxygen sensors will fail soon. You can have them replaced all at once so you don’t have to deal with a bad oxygen sensor again anytime soon.