Symptoms of a Bad O2 Sensor

There are many bad O2 sensor symptoms you may notice.

But first, what is this sensor, and what does it do?

The O2 sensor tells the vehicle computer the status of the oxygen content in the fumes exiting the engine.

It passes important information to the ECU, which can then adjust the air-fuel ratio in the engine. This tool is used for enhancing the fuel efficiency and performance of your car.

The common symptoms of a failed 02 sensor are similar to those caused by the failure of different parts.

Unless you know how to test a bad O2 sensor, it might be hard to know forthright that it is the culprit for the poor fuel efficiency of your car.

Thankfully, while the diagnosis might be hard, it is not impossible.

Two Types of O2 Sensors

These are the downstream and upstream O2 sensors.

A downstream O2 sensor is positioned behind or inside the catalytic converter.

Its work is to give a voltage reading that is changed by the change in the level of unburned oxygen in the exhaust system.

This way, it tells the ECU whether the catalytic converter is working as it should.

The upstream oxygen sensor is located before the catalytic converter. It measures the level of oxygen in the exhaust manifold and sends this info to the ECU.

That way, the engine knows how to adjust the air/fuel mixture. The upstream O2 sensor is more prone to failure than the downstream sensor.

How Does the O2 Sensor Work?

Bad O2 Sensor

The oxygen sensor looks quite simplistic in design.

However, it does a very important job, of monitoring the exhaust fumes from the engine, for changes in oxygen levels.

Thus, it has a sensor tip that is installed right inside the exhaust pipe.

It monitors the amount of unburned oxygen in the exhaust. If the exhaust fumes are too lean, it means there is a lot of oxygen going to the combustion chamber, so the ECU needs to adjust that. If the exhaust fumes are too rich, it means there is too much fuel in the fumes, which affects your car’s fuel efficiency.

Whether the mixture is too lean or too rich, the O2 sensor will send this information to the ECU. It will then know what to do to balance the air to fuel ratio.

Usually, this means that the computer adjusts the percentage of fuel that enters the engine.

What Causes an Oxygen Sensor to Fail?

Many things cause the failure of your car’s oxygen sensor. Some of the most common culprits include using dirty fuel and corrosion. Another common cause is the leakage of antifreeze into your sensor.

The failure of the heating element of the O2 sensor

The oxygen sensor has a heating element. Its work is to warm up the oxygen sensor as soon as you start the car.

That way, the sensor starts monitoring the exhaust fumes as soon as the engine starts to produce them.

If the heating element fails, then ultimately, the sensor has failed because it will not send the right information to the ECU.

Solution: Replace the sensor. It will set you back a couple of hundred bucks, but it will be worth it. It can be hard to replace the heater.

Failed plugs

Failing plugs can be the cause of a bad sensor.

Most of the time, bad plugs will cause fuel that is not completely burned to be sent to the sensor. This will destroy it.

Solution: Always service your vehicle in time, replacing the plugs as soon as the need arises to prevent unburned fuel from getting to the sensor.

Dirt buildup in the exhaust system

Lead, sulfur, fuel additives, and other elements can build up in the exhaust system, thus hindering how the O2 sensor works.

It will not happen to a new car, but as soon as you hit 100,000 miles, the sensor will most likely start misbehaving.

If you have to replace it, we can say that it is a fair deal anyway, since not many vehicle components can reach this mark.

If you continue using the car with the buildup of these combustion elements, the chances are that you could shorten the electrical system.

7 Bad O2 Sensor Symptoms to Look Out For

1. Misfiring becomes a common day thing

Will a bad O2 sensor cause misfire?

It is one of the biggest culprits of a misfiring engine.

In a car engine, a misfire is an indication that one or two steps in the combustion process in the engine have been skipped.

One of the most common indications of a misfire is when the engine stalls for no reason, and then it resumes normal operation.

When the car is idling, you may realize a difference. Sometimes, the idling will be higher, and at times, it will be lower.

Also, if you notice poor speed peaking when accelerating, you most likely have a misfire.

What are the effects of a bad O2 sensor?

Engine misfiring is just one of many. It will require a mechanic to find out what could be the cause of the misfire. It could be spark plugs, defects in the ignition system, fuel system issues, but the most common culprit is the bad O2 sensor.

If there is a problem with the wiring to the oxygen sensor, there is likely to be a misfire. This is why you should know bad oxygen sensor symptoms and dangers.

If your car does not pick speed after accelerating, the O2 sensor should be the first thing to check.

2. Poor fuel economy

If the fuel economy of your sedan drops from 23 mpg to 16 mpg, it means something is wrong.

Chances are there is incorrect air to fuel ratio in the engine, so proper combustion is not happening, so you tend to spend more on fuel.

Remember, the oxygen sensor is designed in such a way that it sends real-time information about the oxygen level in the exhaust fumes to the computer.

Thus, if it is not working, the information that will be sent to the computer will be faulty.

3. Check engine light – one of most common bad O2 sensor symptoms

Most people go to panic mode when they see the check engine light, commonly referred to as CEL.

However, it could be anything, does not mean that the engine is failing. In fact, a failed oxygen sensor is the first culprit that you should check.

What happens to a car when the oxygen sensor is bad?

Many things, but the first of them all is the check engine light popping up on your dashboard. If you see that, take the car to a garage and have a mechanic check things out.

In any case, the first thing they check is the O2 sensor.

4. Engine skips a beat during acceleration

When searching for information on what causes an oxygen sensor to fail, you will find that a failed sensor can cause the engine to stop when you accelerate.

Also called engine hesitation, it is usually a moment of indecision on the engine.

This happens because the computer does not know what to do, as it is not getting the correct information from the O2 sensor.

5. The performance of the car is affected

The performance of the car covers almost every aspect of driving. It covers the fuel economy, which we have already said is one of the bad O2 sensor symptoms. Then there is the acceleration issue as well as engine hesitation.

Another aspect of performance that will be affected is the idling and the engine sound. If you note that the engine is louder than it usually is, has the oxygen sensor checked?

For performance enhancement, the mixture of air to fuel ratio should be at the recommended amounts. If it is not right, straightaway, the performance of the car is affected.

Can an O2 sensor cause a car to cut off?

It might not, at least not directly. If the car starts to stall for no reason, such as an empty tank, then the fuel system could be the problem.

If you notice a change in the color of the smoke from the exhaust pipe, have a mechanic check different things for a proper diagnosis.

Other things you may look out for include jerks that can be hard to note at first, but as they increase in intensity, you will find them very uncomfortable.

6. Nasty smell and smoke from the engine

If you notice a nasty rotten egg-like smell from your car, the sensor has most probably failed.

The black smoke is to be expected because the combustion is not happening, as it should. If you take your car for emission tests, it will fail.

7. Catalytic converter failure – another of the bad o2 sensor symptoms

You will know that you have catalytic converter failure when you start seeing heavier smoke from the exhaust pipe.

The catalytic converter is the metal box that is made of titanium, palladium or other such metal.

Its work is to cleanse the exhaust gases to help your car meet the emission standards.

However, for the catalytic converter to work well, the oxygen sensor must also be working well.

As its name suggests, this is a box with a catalyst which “burns” the exhaust gases further and faster, resulting in the release of harmless gases such as steam, from your car.

As we said, the O2 sensor sends information to the ECU to rectify the air to fuel ratios. Thus, when the sensor sends the wrong information to the ECU, it means that the catalytic converter would have too much work to do. It would fail.

Solutions for Bad O2 Sensor Symptoms

Know how to test a bad O2 sensor

Knowing how to test a bad O2 sensor is the first step towards finding a solution for your car.

After all, the symptoms of a failed O2 sensor can also be caused by other engine parts and processes failures.

Thus, you want to be 100 percent sure that the sensor is to blame.

Some cars have more than two sensors. If yours has three and one sensor has failed, the check engine light will still pop up on the dashboard.

The reason why you should know how to test a bad O2 sensor is so that you can pinpoint exactly which one is faulty.

Here is how to test your O2 sensor with a digital voltmeter

It is not too hard to run this test yourself. First, you should get an impedance digital voltmeter – 10-megaohm.

You should then start the car and leave it running until it reaches the optimum temperature.

Switch the car off and connect the black probe to the ground and the red one to the signal wire of the oxygen sensor. You can then get the engine running again as you check the readings displayed on the voltmeter. If they are within the range of 0.10V to 0.90V, your sensor is working as it should.

So what you have been thinking of as bad O2 sensor symptoms must be caused by something else.

If the readings on the voltmeter fluctuate outside this range, you may have to test the sensor for lean fuel or rich fuel tests.

This is a bit technical, but if you have the tools, it is not too hard. You may start by detaching the hose from the PVC valve.

A lot more air will get to the engine, and the readings on your voltmeter will jump to 0.20V if the O2 sensor is bad.

Instead of going to all this trouble, taking the car to a mechanic is much easier, and they will probably take half the time you would have taken to find the issue.

If the oxygen sensor is faulty, the best solution is to replace it.

After all, sensors mostly fail after covering more than 60,000 miles.

How to test an o2 sensor with the scanner

Knowing how to test the O2 sensor with a scanner is very important for you when preparing for that emissions test. It is also important if your car has been misfiring, hesitating during acceleration, and displaying some or all of the symptoms that we have discussed here.

If you have your scanner tool, and there are many of them in the market, you can follow the following steps:

To start, insert the scan tool into the car’s DLC – diagnostic link connector. It is easy to identify it since it looks triangular, and it has 16 pins. You may also look at your car’s manual to find it.

Switch on the car, but do not fully start the engine. The scanner should pop into life if it is working correctly. If it does not, shake it a bit. You might have to enter the VIN – vehicle identification number, but this depends on the scanner. Once the scanner has fully booted, you can check codes or trouble codes on its menu.

You can choose the codes for troubleshooting.

These are Pending or Active Codes/Live codes that cause the Check Engine Light to show on the dashboard. If it displays a pending code, it means that the emissions system has failed once and that another failure can cause the CEL to light up.

When you get the codes from the scan tool, you will have to interpret them.

You can do that online and see what the problem is. You now know how to test 02 sensor with a scanner.

If you want to know how to test the O2 sensor with the OBD scan tool, follow this process. It works the same way.

How to test an o2 sensor with 4 wires

Knowing how to test the O2 sensor with 4 wires is important if you would like to save yourself some money.

It is not too hard.

But first, you need to understand the anatomy of a 4-wire sensor.

The first two wires heat the O2 sensor so that it does not have to wait for the engine to warm up so it can warm up. This means that as soon as you switch on the car, the sensor starts sending information to the ECU. The 2 other wires are for the power and signal from the computer or the sensor itself.

Check the voltage of the wires with a voltmeter.

You should check the voltage when the engine is warm, with the key in the ignition, but the engine should not be running.

You should do this with the engine cold and warm and ensure the readings are the same.

If they fluctuate between 0.10V and 0.90V, the sensor is working correctly when the engine is warm.

To check the ground wires voltage, if it is more than 0.25V, there is a short, which should be checked out first.

How to test the o2 sensor heater circuit

The O2 sensor heaters fail sometimes. Usually, this happens because of age.

When that happens, you need to know for sure, so that you can think of replacing a O2 sensor. That is why you need to know how to test the O2 sensor heater circuit.

When you carry out a diagnostic test, you will get the code P0135. This is the code for a malfunction in the oxygen sensor heater circuit.

Inside the sensor, there is a heater. This heater can send a code that can light up the check engine light on the dashboard.

In fact, some vehicles have had their O2 sensors replaced only to display the same heater malfunction code a few days later.

In that case, it would be better to test the O2 sensor heater circuit.

One of the ways to do this is to test the O2 sensors heater circuit amperage.

This test is going to show you whether the heater is working. But the test does not stop there. It goes on to establish whether the sensor heater is switching on at the required time.

While this shows you whether there is a problem, it will not show you where the problem is.

Thus, you have to carry out a more advanced test.

Test O2 sensor heater circuit with a multimeter

Turn the car ignition key to lock and then disconnect the Air-Fuel sensor connector. You can measure the sensor heater element resistance.

If the heater circuit is good, the multimeter should display a reading of between 2.5 and 4 Ohm. If it displays anything outside that, the sensor needs to be replaced.

Bad O2 Sensor Symptoms FAQs


What happens to a car when the oxygen sensor is bad?

Your car starts to lose its efficiency both in performance and fuel economy. In fact, poor mileage is one of the bad O2 sensor symptoms. You will also experience poor idling, jerking, and engine refusing to respond promptly when you accelerate. The check engine light never goes away.

Will a bad oxygen sensor cause car not to start?

No, the car will start. It may experience a hard start, but it will start. Usually, the causes of no-start problems in the car include the starter/ignition system (coil, starter, pick up, module, and so on) or the fuel pump.

Can an o2 sensor cause a car to cut off?

The O2 sensor is NOT going to cause the engine to stall. However, when the O2 sensor problem is related to another engine problem, the car can stall. Thus, the car will not stall as a direct relation to the oxygen sensor. Check the fuel gauge though because a bad O2 sensor can affect the fuel economy drastically. It is one of the bad O2 sensor symptoms.

What are the symptoms of a bad downstream O2 sensor?

The downstream O2 sensor will have failed if it starts to mirror what the upstream sensor is doing. The downstream sensor is supposed to measure the efficiency of the catalytic converter. It will send info about the level of impurities in the exhaust fumes coming through the converter to the ECU where a comparison will be done with the info sent by the upstream converter.

What happens if you don’t replace O2 sensors?

The vehicle computer is not going to signal the engine to correct the air/fuel ratio. Your fuel economy is not going to improve, and the car will jerk, produce a lot of smoke, and fail emission tests.

The car will move, but you will most likely be stopped for polluting the air or driving an unroadworthy vehicle.

When should I replace my oxygen sensors?

If you are asking this question, it means yours haven’t failed yet. However, with age, material buildup, and usage, O2 sensors fail. It is recommended that you replace your car’s O2 sensors if your mileage is between 60000 and 90000 miles for older cars. If you have a newer model, check for bad O2 sensor symptoms such as jerking,

What are the bad oxygen sensor symptoms and dangers?

Even with a bad oxygen sensor, your car will still run. Some people can spend even a year with a bad O2 sensor light on the dashboard for a year. While a bad sensor might not stall the car, it can cause bigger problems to other parts of the engine. It is like knowing you are sick, yet you avoid going to see the doctor.
Jason Hennessey

I have lived and breathed the automotive sector for over 7 years. As a taxi driver, fleet owner, garage services provider, I have gained the knowledge and passion for providing a service that is fair for everyone involved in the ride-hailing industry.

1 thought on “Symptoms of a Bad O2 Sensor”

  1. Thank you for the info it was very helpful. My o2 sensor does not look good.I have 233.000 miles on my nissan exterra never had it changed hope it hasen’t caused any other problems.


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