Bad O2 Sensor – How To Recognize The Symptoms?

Bad O2 Sensor – How To Recognize The Symptoms?

July 31, 2021 0 By CarGuy

So, recently you started experiencing some problems with your car. And these symptoms are like bad fuel efficiency, black smoke out of the exhaust, and a check engine light? You shouldn’t worry, probably you are having a bad O2 sensor.

Almost every driver is experiencing this issue every once in a while. O2 sensors don’t fail very often, but when they do fail, they will make your car almost undrivable. When the O2 sensor is failing there are a ton of symptoms and your car feels ill.

When you are feeling that your car is not in proper working order. Тhe best thing to do is to pay a visit to your local mechanic’s shop. These things must be resolved before they develop into something more serious that could potentially damage your engine in the long run.

The O2 sensor is an important sensor that measures the exhaust gasses. This sensor communicates with the car’s computer and tells it what to do. If these readings are wrong you are going to have a hard time. But you shouldn’t worry.

In this article, we are going to cover everything you need to know when it comes to bad O2 sensor. Namely the bad O2 sensor symptoms. You need to know how to recognize a broken O2 sensor first. Then you can start troubleshooting. We will also learn how to test a bad O2 sensor. How to find a temporary fix for a bad O2 sensor. And we also going to learn how to replace an O2 sensor and what is the cost of an O2 sensor. So, if you want to learn more about bad O2 sensors, follow along.

What Is An O2 Sensor?

An O2 sensor also known as lambda sensor is a device that is installed in the exhaust system. It is placed right after the catalytic converters. And the role of this sensor is to measure the levels of O2 or oxygen inside of the exhaust gasses.

This sensor works like a probe that tells the ECU which is the brain of the car how to adjust the air to fuel mixture. In order for the catalytic converter to work at optimal values.

The O2 sensor is a crucial component in modern fuel injection vehicles. Without an O2 sensor, there will be no proper fuel combustion. These O2 sensors make the car work efficiently and without problems.

The sensor works in a way that it measures the difference between the oxygen inside of the exhaust gas and the amount of oxygen in the air. If your car is running rich, it will require oxygen demand and will cause the voltage to build up inside the sensor. If there is a lean mixture that will cause low voltage in the sensor, this is because there is plenty of oxygen.

This probe was first installed on cars back in 1976 and is still in use to this day. The sensor is mounted right behind the catalytic converter. It is screwed inside the exhaust from one end and on the other end is connected to the wiring harness.

Bad O2 Sensor Symptoms

Like with every other part of your car, when an O2 sensor goes bad it develops symptoms. Some of these symptoms are a dead giveaway that you are having a bad O2 sensor. Some of them are not that clear. We are going to study them one by one and see what are their characteristics. So, let’s begin.

Check Engine Light

If you have a bad O2 sensor, the check engine light will be there to notify you. The car computer is smart and knows when a component, especially a sensor is failing. When the sensor is failing you will start to notice a check engine light on the dash in most situations.

This check engine light will be a constant reminder that you need to take your car to a mechanics shop in order to get it fixed. If you don’t do that, it can get worse and you may end up paying more. Because if some components do not work properly and they are not fixed with time. These components may damage other components. And if your O2 sensor does not work it can put some stress on the ignition system and spark plugs because there are going to be misfires.

You can also investigate this issue by yourself and get an OBD2 scanner tool. You can get these for an affordable price online on Amazon or at your local car parts shop. They are fairly inexpensive and they can tell you almost everything about your car.

You just plug the scanner into your OBD2 port and scan the car, if there is a bad O2 sensor you will be notified. You will have to search online for the codes if you are having a simple scanner. More advanced scanners will tell you the problem right away. So, getting a more advanced OBD2 scanner will tell you a lot about your car.

Since you diagnosed the issue, the next thing is to replace the bad O2 sensor. How you can do that we are going to explain at the end of the article.

Engine Misfires, Will A Bad O2 Sensor Cause Misfire?

Yes, it will cause your engine to misfire. Misfires are one of the common occurrences when you are having a bad O2 sensor. You will notice if your engine misfires judging by the work of the engine. If your exhaust is making pops from behind and does not work evenly. This is a clear sign that your engine is misfiring.

This is happening because of the bad readings that this sensor has. The malfunction of the sensor is giving the computer bad readings and the car computer is not being able to adjust the right fuel to air ratio.

Bad fuel to air ratio is going to cause the engine to misfire and possibly foul your spark plugs. These misfires can cause carbon to buildup on the spark plugs and make these misfires even more pronounced.

So, the best thing to do is when your engine misfires is to see the spark plugs and their condition. Then further investigate what could possibly damage the spark plugs? And there you are going to find your answer.

Significant Drop In Power

Drop in power is another common symptom of a bad O2 sensor. If this sensor breaks your engine will not work well, and this can damage your power ratings.

Sometimes, your car can even get into limp mode. A limp mode is a special program in the car computer that when it detects a big issue it does not allow the car to increase its speed above 20 mph. You will have enough power to limp your car to the closest repair shop in order to get it fixed.

If you have big drops in power. Then these issues need to be investigated and fixed on time. Because they can even possibly damage your engine components.

Increased Fuel Consumption

If your O2 sensor is failing then you are most probably also have increased fuel consumption. Personally, I had this issue with my vehicle. My O2 sensor was bad and I frequently visited the gas station. The back of my exhaust smelled of unburnt fuel until I replaced the bad O2 sensor.

How this works is pretty simple, the bad O2 sensor is telling the computer to dump more fuel into the combustion chamber and the engine does not have the time and spark to ignite all of the fuel. So, this not-ignited fuel is dumped into the exhaust causing misfires and exploding sounds when your car is running.

The only way to fix this issue is by replacing the O2 sensor. Later in this article, we are going to explain how you can do this.

Blue Smoke From Exhaust

Blue smoke is another symptom of a bad O2 sensor. The car is dumping so much fuel inside of the combustion chamber and this fuel does not burn completely.

It creates clouds of blue smoke when you apply the throttle. If you are experiencing increased fuel consumption as we mentioned, plus blue smoke coming from the exhaust. Then you most probably have a bad O2 sensor.

The blue smoke should not be confused with black smoke. If you have black smoke coming from the exhaust it means that your car is burning oil. So, keep this in mind.

Increased Emissions

Increased emissions are also one of the symptoms of a bad O2 sensor. When there is dumped a lot of unburnt fuel into the exhaust and the fuel is starting to ignite in the exhaust. Your emission readings will go mental.

Most probably you will not be able to pass the inspection. At the inspection, your car needs to follow some standards. And if your car is burning too much fuel your readings will be very high. The workers at the inspection will tell you to fix your car and bring it back for registration.

Sulfur Smell Coming From The Exhaust

Sulfur smell or rotten egg smell as most people know this symptom is happening when the Sulphur that is in the gas is released into the atmosphere.

Every fuel contains a small amount of Sulphur. The catalytic converter’s role is to dissolve this Sulphur and convert it into something less toxic.

So, if the O2 sensor doesn’t do its job there will be unburnt fuel into the exhaust and this unburnt fuel will cause some blue smoke and rotten egg smell.

If you notice one of these symptoms you need to act quickly and resolve this issue.

Engine Running Rough

Rough idle is also one of the symptoms that will haunt you when you are having this issue. The rough idle will be caused by the misfires that are happening inside of your engine. These misfires will cause some really rough idling.

The tachometer needle will start to shake and move up and down. When this happens then there is clearly something wrong with your car and needs to be inspected and properly addressed.

If you have a rough idle you are also going to get a check engine light. The best thing when you are having this problem is to further inspect this issue on a car scanner and find the real problem.

Engine Stall

An engine stall can also happen, this is accompanied by the rough idle as we mentioned above. The engine of your car will possibly stall and you will have problems with starting your car.

If your engine is trying to stall you should always give a little more gas to avoid stalling. But more gas means more unburnt fuel dumped inside of the combustion chamber and this translates into more money spent on fuel.

The best thing when your engine is stalling is to try to fix the issue. Don’t delay it because sometimes you will need your car and you won’t be able to start it. And that can be a big headache.

What Does Bad O2 Sensor Looks Like?

Depending on the culprit that causes the O2 sensor to fail, there will be different colors on the sensor. This is important to notice because these colors will represent some issue inside of the engine that needs to be fixed.

If the sensor is all black it means that your car is running rich on fuel. You need to adjust the air to fuel mixture in order not to ruin your new sensor.

Another thing can be coolant contamination. If you have coolant contamination the sensor will be all white. So, why coolant inside of the exhaust? Well, if you have a blown head gasket the coolant will burn in the combustion chamber and then will travel inside of the exhaust. This means that you need to fix your head gasket.

If you have Lead poisoning, the O2 sensor is going to be orange. This is caused by bad gas that contains a lot of Lead. Modern fuels should not contain lead, but in places where fuels are not that regulated, this can happen to you also.

Will A Bad O2 Sensor Cause Car To Shut Off?

A bad O2 sensor will not cause your car to shut off. Your car will keep work but it’s going to be very unhappy. Your system will probably malfunction and you will get all sorts of issues with your engine. What issues? Check the symptoms above. If your sensor is malfunctioned or is not installed you will get misfires, rough idle, blue smoke, loss of power, and the car shifting in a limp mode in some cases in order to protect the engine.

Will Bad O2 Sensor Cause Misfire?

Yes, it’s going to cause the car to misfire badly in some cases. That is basically one of the main symptoms of a bad O2 sensor. How you can identify a misfire? Well, give it a little rev and if you hear some bangs coming from the exhaust your engine is probably misfiring.

How To Test A Bad O2 Sensor?

In order to test a bad O2 sensor, you are going to need a multimeter. A multimeter is a tool that will allow you to measure the resistance and voltage. You can test the sensor in two ways.

The first way is to measure the resistance. You turn your multimeter on Ohms and you place the probe on the connections of the sensor. If there is resistance it means that the sensor is in good condition. If there is no resistance it means that the sensor is bad.

Another way is to do this by measuring the voltage. In order to measure the voltage, you need to apply heat to the probe of the sensor. Make sure you heat up really nice and then check the connections. If the sensor is good it’s going to produce some volts. If the sensor is bad it’s not going to produce anything.

How Do You Clean O2 Sensor To Make It Work Again?

So how do you fix a faulty oxygen sensor? The short answer is by cleaning it. You will have to remove the sensor and then you need to clean it.

The sensing element is closed with a metal cover so it will be difficult, but there are some tricks that you can pull out. So how to trick an O2 sensor? And these tricks include placing the sensor in a carb cleaner or brake clean. Just submerge only the sensing element and not the whole sensor. After that apply a big pressure with carb cleaner in the small holes and the dirt should come out.

This is a good temporary fix for a bad O2 sensor. It will not last forever and you will need to repeat the procedure soon. Just remember that will not always work, especially if your sensor is dead.

Faulty Oxygen Sensor Repair Cost

O2 sensor fix cost will cost you around $60 for the part alone. Sometimes these sensors can cost even more, depending on the make and model that you have. Because not all cars are the same.

The labor is probably going to cost you $200-$300 if you go to a reasonably priced shop. If you take your car to some fancy shop, they are going to charge you a lot more. This is a fairly simple job that only requires a lift and unbolting the O2 sensor and replacing it with a new one.

You can also do this job at your home. But remember that you are going to need a lift in order to lift the car and reach the sensor. If you have done this before and you think that it’s no biggie you can replace it by yourself.

Then you just need to wipe out the codes and you are good to go. No more sensor issues for you.

Are All O2 Sensors The Same?

O2 sensors are not the same. Every manufacturer has a specific sensor for each vehicle and these parts are not interchangeable.

These sensors are different for most models and if you want to replace your existing sensor you will need to find a proper replacement part.

You can do this searching by your make and model, also if you have the parts number you can search by the number.

Can A Car Run Without O2 Sensors?

A car can run without O2 sensors, but it’s going to be extremely angry and you will have to find a way to fix this problem as soon as possible.

The computer will know that there is no sensor plugged in and will throw a check engine light on the dashboard.

If you want to run a car without O2 sensors you will have to buy a carbureted vehicle. Carbureted cars do not require computers, sensors, injectors. Everything is done without any complicated technology. You just adjust your carburetor and you are good to go.

What To Do After Replacing Oxygen Sensor?

After replacing an O2 sensor it is a good idea to plug in your OBD2 scanner and scan the car for codes. Make sure that you erase all the errors and make sure that the car is happy with the new sensor.

If the new sensor is not good or you failed to install it properly you are going to be reminded with a new check engine light.

When installing these sensors be sure that you are doing everything according to the manual and follow the instructions very carefully.


In this article, we have covered a lot when it comes to a bad O2 sensor problem. We discussed all the important symptoms that these sensors have when they are going bad.

If you have frequent misfires, check engine light, blue smoke, and bad smell coming from the exhaust the most probable reason is a bad O2 sensor.

Then we have discussed all the important aspects of bad O2 sensors. We discussed how you can clean this O2 sensor, how you can replace it, and at what cost. Then we answered some frequent questions that bother people who have a faulty O2 sensor. Thanks for reading and if you like our article, you can follow us on social media to be readily updated when new articles are coming out.