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Bad Oxygen (O2) Sensor: Signs, Symptoms, and Replacement Cost

Sometimes the smallest things can cause the largest problems, and in the case of a failing O2 sensor, this certainly holds true. Because it plays such a vital role in making your engine and emissions systems function properly, poor fuel economy, poor engine performance, rough idling, a failed emissions test, and even vehicle stalling all could occur if your vehicle’s O2 sensors have gone bad.

What Is an O2 Sensor?

oxygen sensor

An O2, or oxygen sensor, is a device fitted to a vehicle that communicates oxygen levels within the exhaust to the vehicle’s computer – its ECU. O2 sensors are usually located somewhere along the exhaust system after the catalytic converter for vehicles with only one sensor. Vehicles with two usually position an additional sensor between the exhaust manifold and the catalytic converter.

This sensor plays a vital role in both emissions and engine performance, communicating to the vehicle’s ECU what the proper air-fuel mixture should be for optimal engine combustion.

Common Signs and Symptoms of a Failing O2 Sensor

Seeing as how the O2 sensor plays such a large role in ensuring that the engine runs properly, it is easy to understand how many of the signs and symptoms of O2 failure are related to the engine’s suboptimal performance. However, there are other signs and symptoms of sensor failure as well.

Emissions Test Failure

emissions test failure

The O2 sensor serves as an emissions device first and foremost. If it does not function properly or is completely broken, your vehicle will not be able to pass an emissions test. Not all states require emissions testing and some states that do only require them for parts of the state. All this to say, an emissions test failure will not always detect O2 sensor malfunction since it is not universally required.

Poor Fuel Economy

A drastic drop in fuel economy is one of the telltale signs of oxygen sensor failure. The EPA estimates that fuel economy could drop by up to 40% when an O2 sensor fails. This is because the necessary communication of the proper air-fuel mixture within the engine is either hampered or incorrect. The result is the drastic loss of efficiency.

Engine Overheating

One of the less common issues associate with O2 sensor failure is engine overheating. This occurs when too much fuel is consistently used for combustion. Since the oxygen sensor is the primary regulator of the air-fuel mixture, overheating will continue occurring until the sensor is replaced.

Thankfully, there are many other systems at work in most vehicles. These systems can usually keep the overheating issues at bay for a while, but consistent overheating will eventually cause problems, and the other vehicle systems will not be able to keep up. 

Rough Idle

A rough idle is probably going to be one of the first signs of O2 sensor failure. It is the direct result of a failing or already failed sensor since the optimal combustion mixture can no longer be controlled. Two phrases used for this are “running rich”, and “running lean”. Running lean means an excess of air within the air-to-fuel ratio, and running rich means an excess of fuel used within the ratio. 

Both issues lead to improper combustion which can cause engine hesitation as it struggles to nail down the combustion “sweet spot.”

Poor Engine Performance

If your vehicle’s engine is suffering at idle, it logically follows that the issues will translate well when you start moving. Overall engine performance when you are driving, including acceleration, staying at a consistent speed, and hesitation at any time are all the result of the same issues that cause rough idling. 

Even though poor engine performance, rough idling, and engine overheating are all very similar and somewhat connected symptoms of a bad O2 sensor, each represents an escalation in severity and the greater potential for overall engine damage. Poor engine performance and overheating can be caused by other things, but experiencing any of these signs and symptoms should warrant a trip to the mechanic. 

Misfiring

An engine misfire – incomplete or total lack of combustion within one or more combustion chambers – is a two-headed monster. It can be the cause of poor overall engine performance and rough idling or it can be the culmination of prolonged engine performance issues. Either way, a misfire is a sure sign that your vehicle needs to be inspected. 

It can sometimes be difficult to diagnose an engine misfire since it usually feels like hesitation or an overall lack of power. This is why it is so important to have your vehicle inspected if it starts to feel abnormally sluggish.

Vehicle Stalling

Vehicle stalling is even one step higher than an engine misfire. With a misfire, an engine can still work and power a vehicle, albeit on a lower level than when it is running well. Vehicle stalling usually occurs when a misfire is so bad that the engine can no longer sustain itself with the remaining working pistons. Of course, a failed O2 sensor, both directly and indirectly, can cause engine stalling in worst-case scenarios.

Again, many of these issues, including stalling, can be signs of other issues. A good way to better pinpoint the O2 sensor as the cause of poor overall engine performance is to replace your vehicle’s spark plugs. They are a very common cause of these issues as well, and if a change does not fix the issue, it is a good sign that the oxygen sensor might be the culprit.

Check Engine Light

As with most vehicle system issues or failures, the check engine light can come on in the event of an O2 sensor failure. But, unlike many other systems, oxygen sensor failure does not always directly cause the check engine light to illuminate. More often, other systems fail as a result of the O2 sensor failure that causes the check engine light to illuminate. 

When this happens, it should not be ignored. Most automotive retail locations offer free diagnostic testing, a good option for anybody who does not have their own OBD scanner.

What is the Expected Lifetime of an O2 Sensor?

o2 sensor in exhaust pipe

Oxygen sensors are expected to last from about 100,000 miles to 150,000 before normal wear and tear get the best of them. Older sensors on older vehicles did not last as long as the newer, more technologically advanced sensors on newer vehicles.

There are usually a few things that contribute to premature sensor failure, most of which have to do with damage to the sensor itself. Any debris, like carbon buildup from the engine, can blow by the sensor and damage it. Additionally, any other physical damage to the area in which the sensor resides like road dirt and grime can damage the sensor and cause it to fail.

Other causes for premature failure are contaminated, bad quality fuel, and too much oil being burned by the engine. Both can cause buildup on the sensor and “clog” it. When this happens, it will stop measuring correctly or entirely.  

How Much Does an O2 Sensor Cost to Replace?

Fortunately, an O2 sensor is usually not that expensive. They usually range anywhere between $30 and $100 for the cost of the actual device. Having a mechanic replace a bad sensor will incur additional labor costs. These costs will depend on the make, model, location, and the number of sensors being replaced. Newer vehicles often have more, harder-to-reach sensors, potentially making labor costs higher.

If you choose to replace the sensor yourself, it will only cost what the sensor does. Having a mechanic do this can cost anywhere from $150 to well over $400 depending on the combination of all these factors. 

Final Thoughts

Your vehicle’s O2 sensor is an important piece of equipment, necessary for emissions purposes and to keep your engine running optimally. Because it controls the air-fuel mixture your engine needs to run well, a bad oxygen sensor will cause decreased fuel efficiency, rough idling, poor overall engine performance, and even stalling if starts to fail or have already done so. It is important to have your vehicle checked if these signs and symptoms start to exhibit themselves.

Shawn Furman
I have been a vehicle hobbyist for as long as I can remember as well as a freelance writer for the past three and a half years. My clients have included Vehicle Scene, Autolist, CarGurus, and now The Vehicle Lab. In addition to my current clients, I also maintain my own blog where I am able to share my knowledge and experience through vehicle reviews, car-buying guides, how-to guides, and list articles.

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