Knowing and recognizing bad O2 sensor symptoms on time is of the utmost importance for the proper functioning of your car.

Once the sensor starts to malfunction, it affects other systems which can eventually cause bigger and more expensive damage.

In this article, you’ll see what are some of the most common bad O2 sensor symptoms as well as some other related information that may come in handy.



What eyes, ears, or the nose are to a human, the sensors are to the ECU unit (Engine Control Unit or the car’s computer so to say).

The task of the ECU is to monitor and configure all of the cars vital functions while it’s running in order to assure the proper functioning of the car.

Sensors play a key role in this system and without them, the ECU would be more or less ˝blind˝.

Such is the case with the O2 sensor.

The O2 sensor is an electronic device that measures the amount of oxygen in the exhaust gases that come out of the engine via the exhaust manifold.

This sensor was implemented mainly with the environment in mind.

Constantly monitoring and adjusting the air-fuel mixture ratio meant a near-ideal combustion. This, combined with the catalytic converter meant clean exhaust fumes (or as much as possible).

While measuring, the sensor sends electronic impulses to the ECU which then adjusts the air-fuel mixture that goes into the engine. If the mixture is to lean or too rich, the O2 sensor sends signals and the ECU makes the needed adjustments.

This is an explanation in a nutshell. If you have the time and will, click here for a more thorough explanation from Wikipedia about the purpose of the O2 sensor, what it’s made out of, etc.

If you like a more simple explanation about the O2 sensor: it’s essentially a big, partially hollow screw located on the exhaust manifold with a couple of wires sticking from it. Its main purpose is to measure what the engine exhales while running.

In most cases, there are two of them (even more for V-type engines). They are usually located before the catalytic converter (on the exhaust manifold) and after it.

One more piece of useful information: other common names for the O2 sensor are:

  • Oxygen sensor
  • Lambda sensor

Also, the O2 sensor is exclusively for the gasoline engine (no O2 sensor on a diesel) for the readers with less car-knowledge.



These are some of the most common bad O2 sensor symptoms on most cars:



Yes, the infamous yellow, weird-looking dashboard warning light. The most hated one also, I should say.

The O2 sensor is one of the most common reasons for this light coming up on the dashboard.

Once the engine starts getting an incorrect air-fuel mixture the ECU will react by lighting up the check engine light.

I must say on this part that the check engine light will come up for various other reasons that affect the O2 sensor and render it faulty although it’s OK (like loose gas tank caps, loose air intake hoses, etc).

All of these seriously affect the functioning of the O2 sensor as they interrupt the proper air-fuel mixture ratio.

Even a diagnostic OBD 2 tool will give you a faulty oxygen sensor code while the malfunction may be caused by something else.

This can be a huge mislead that can cause unnecessary repairs and cost.

If you’re interested in more check engine light topics you might want to look at these articles:



One more of the major bad O2 symptoms that you’ll notice soon is very poor or fluctuating fuel consumption.

It’s all due to the variations in the air-fuel mixture as a bad O2 sensor sends wrong input to the ECU.

In one moment the mixture is to lean, in another, it’s too rich.

This leads to the engine working improperly and inevitably heightens the fuel consumption.

The car may use substantially more fuel: 1 liter to even 3 liters of gasoline more on every 100 kilometers.

You’re bound to notice this quickly, either through a steep fall of the fuel gauge needle or through calculating the fuel consumption.

If you want to learn more about how to calculate your fuel consumption, click here.



You’ll notice this symptom once you start the car.

The idling won’t be as smooth as usual and will seem like the car is not running on all cylinders.

This symptom may go away as the engine heats up.

Just know that, in most cases, as the O2 sensor fault gets worse so will the idling.



Poor acceleration is among one of the more obvious bad O2 sensor symptoms.

It’s caused, in most cases, by the ECU activating the so-called limp mode or safe mode.

Almost all cars have this feature and its main purpose is saving your car from further damage.

In this case, a bad O2 sensor will give an incorrect signal to the ECU that the air-fuel mixture isn’t correct.

Automatically, the ECU switches on safe-mode, and in most cases, the first step is reducing the engine power output. It’s usually accompanied by the appearance of the check engine light.

You’ll notice it as the car will become sluggish and responds poorly to the accelerator pedal. It will seem like the car functions at 50 percent capacity.

While the O2 sensor still partially functions, the car may not go in to limp mode immediately but you’ll notice that the engine isn’t ˝pulling˝ as it normally should.



When talking about bad O2 sensor symptoms, engine stalling occurs less than rough idling or poor acceleration.

If the engine tends to suddenly cut off while idling or when you press the accelerator pedal the sensor may be the problem.

It’s usually accompanied by the check engine light flashing up on the dashboard.

This is one symptom that usually appears in the later stage of the O2 sensor fault.

Once more, like with the check engine light problem, be sure to pinpoint the exact cause of stalling before accusing the O2 sensor.

Best make a good diagnostic as there are various other things that can cause engine stalling.



I’ve had this symptom myself and it mostly appears once you start the car. It’s especially noticeable when the engine is cold.

It’s a smell of ˝raw˝ gasoline coming from the tailpipe, just like if there was an open container of gasoline near the car.

The main reason is a rich air-fuel mixture (in this case more gas than air).



A proper air-fuel mixture combined with a functioning catalytic converter is key for passing an emission test.

Once the test probe is hooked to the exhaust pipe for the emission test, an improper air-fuel mixture (especially a rich one) will give bad, no-pass results.

Maybe a good side of this symptom is that it can be a tell-tale sign of O2 problems to come.

If the results are borderline on a previous emission test you may want to check the O2 sensor although there may be no problems yet.



  • Make sure that the engine combustion is good

This means that the air intake, fuel intake, ignition system, and else should always function with one hundred percent capacity.

If not then a build-up can appear on the sensor (caused by a bad combustion) which can cause the sensor to lose its ability to send a proper signal to the ECU.

  • Don’t use poor quality fuel

This is one of the most common reasons. Using poor quality fuel will both cause improper sensor readings and a build-up of filth on the sensor itself.

  • Don’t use improper fuel

Always use the gasoline type and grade that the car manufacturer recommends.

For instance, using gasoline with the wrong octane value can damage the O2 sensor in the long run.

  • Do the car maintenance on time

Not changing things like the spark plugs, air filter, fuel filter, having cracked air intake hoses, etc. will all cause problems with the air-fuel mixture, combustion, and exhaust gases which can lead to O2 sensor problems.

  • Use fuel additives moderately

Using fuel additives for a prolonged period of time can damage the sensor as some additives seriously influence the air-fuel mixture and thus the exhaust fumes.

Use every fuel additive wisely and as long as the manufacturer recommends. Also, make sure to use the recommended amount per full gas tank.

  • Driving style

Now this one is a highly subjective suggestion and is certainly not obligatory.

Gasoline engines are not very fond of bogging down. You know they like a bit of revving up from know and then. This is simply one of the characteristics of the engine.

If you’re driving style is the sheer opposite of ˝pedal-to-the-metal˝, then give your car a bit of a rev-up from know and then. The highway is the best place to do that.

Bogging down the car will over time increase the possibility of a filth build-up on the O2 sensor.



Bad O2 sensors are one of those car problems that most drivers tend to postpone until more serious problems occur.

These symptoms will rarely cause any immediate damage, but over time, if left unattended, they can cause damage to the engine and other systems mainly through improper air-fuel mixture.

Then there is the matter of irregular engine functioning like stalling, poor acceleration, and else which endangers your safety while driving.

Not to mention the cost of higher fuel consumption and the matter of ecology and environment.

So, the moment any of these bad O2 sensor symptoms appear make a good diagnostic and if you see the sensor is the problem, replace it as soon as possible.

You also have the option of various cleaners for the O2 sensor in forms of sprays or dilutions that clean the carbon and grit build-up on the inner part of the sensor.

What can I say? The cost of a new sensor is pretty high (50 to 100 Euros on average; parts + labor) so it’s worth the try. Just have in mind that nothing can certainly beat a new sensor.

So, once you notice any of these bad O2 sensor symptoms it would be best to react as soon as possible and solve the problem.


Written by: Sibin Spasojevic


Former car technician, life-long car and DIY enthusiast, author for