Bad O2 (Oxygen) Sensor Symptoms


One of the sensors on your vehicle is the oxygen sensor. While this particular part may be unfamiliar to you, it plays an important role in making sure your car runs smoothly because it monitors the air to fuel ratio. 

Bad O2 sensor symptoms include a bad fuel economy, a failed emissions test, misfiring, the check engine light comes on, black smoke out the exhaust, an engine ping, and weak performance.

It’s important to pay attention to these signals because a failed O2 sensor doesn’t have an obvious sign, like a stalled engine, that something is wrong. Rather, it’s several little symptoms that will clue you in. Read on to find out more. 

What Does the O2 Sensor Do?

The oxygen sensor is located in the exhaust system. Its job is to measure the oxygen leaving your engine through your exhaust. It sends the information to the Powertrain Control Module (PCM) and the PCM determines the right air to fuel ratio for your engine

The result is the engine timing and fuel injection works well. That helps with fuel economy controlling emissions. 

When the O2 sensor fails, some of the first things you will notice are a loss of fuel efficiency and perhaps a failed emissions test.

What Are The Symptoms When O2 Sensor Goes Bad ?

Bad Fuel Economy

Your car works on a set system of ratios and balances set by the computer. A fault oxygen sensor can throw all that off and disrupt the air to fuel ratio. What happens then is too much fuel gets pumped into your engine and that reduces your gas mileage. 

If you are noticing more trips to the gas pumps, start keeping a record of your fill-ups. This could tell you if there is a problem. 

Failed Emissions Test

This is a no-brainer and one you can’t ignore since most cities require an emissions test to get tags and registration. Remember, the O2 sensor measures oxygen going out with exhaust gases which are then used in the air to fuel ratio. If it is failing, those numbers are going to be off and result in a no-go on the emissions test. 


A misfiring engine, a rough idle, or a rough riding car can point to any number of problems from bad gas to any of the sensors going bad. Many of these sensors, including the oxygen sensor, affect the engine timing and combustion so that is why the common symptom is misfiring, rough idle, or jerky movements. 

One of the other things your vehicle may do with a faulty O2 sensor is hesitation. Hesitation happens only when you try to accelerate. This is usually a sure sign of a bad O2 sensor.

Whenever the engine function and combustion are interrupted, the result is going to be a terrible idle and a rough ride. 

Check Engine Light

The check engine light is an all-purpose dummy light to get your attention that something is going on with your vehicle. Like misfiring or a rough idle, a check engine display can mean any number of things but none of them are good. 

Your vehicle will produce several internal error codes when the check engine light displays. These codes can help tell you more specifically what the problems are.

A scanner is required to see the codes. The code that typically pops up for an O2 sensor is P0420. That isn’t much of a help though because it is a more generic code and could relate to several issues.

You may also get a P0136 code. This is also a generic code but is more specific as it indicates something is wrong with o2 sensors on the first bank of the exhaust system. The typically meaning behind the code is the sensors are either dirty or bad.

Your vehicle may ride okay for a while with the check engine light on but that won’t last forever. This is especially true with a failing O2 sensor. You can’t continue to drive without the right air to fuel ratio. It’s simply a matter of chemistry and engineering. It will get worse over time.

Black Smoke

Black smoke coming out of your exhaust not only tells you something is wrong but tells the whole world driving next to you. It means there isn’t proper combustion. You need three things for proper combustion: air, fuel, and fire. 

A failing O2 sensor is preventing air from getting where it needs to be and that is also preventing the right amount of fuel from coming into the engine. The fire then has trouble igniting. 

A poor combustion results in black smoke. The smoke is the residue left in your exhaust. 

Engine Ping

Another problem that stems from improper combustion is a pinging sound in the engine. A failure of the O2 sensor messes up the advanced engine ignition timing. That can cause the engine to make a pinging sound. 

The pinging sound is one of those symptoms that are unique to the failure of an oxygen sensor. 

Your engine may make other unpleasant sounds as the oxygen sensor fails. This is because of improper combustion. A lot of strange sounds coming from the engine should send you straight to your mechanic to have it checked out.

Weak Performance

The weak performance is another one of those symptoms that can mean any number of things. In this case, it is the result of poor combustion. It just won’t go as fast or as quickly as you want it to go no matter how hard to step on the gas pedal. 

A bad O2 sensor can certainly be blamed for weakened performance. The sensor is responsible for monitoring the air, which then leads to the correct fuel ratio. 

A failing oxygen sensor will send wrong messages to the control module and so an improper amount of fuel is released to the engine. That will result in low performance.

This is perhaps the most serious of all the symptoms for an O2 sensor because it could cause safety issues in traffic. You need your vehicle to perform well when you are merging into traffic, making a turn, or trying to pass another in a lane. 

One thing you should check on your vehicle is whether it has a “limp mode.” Many cars today have this feature that shuts the engine down some when the car computer senses something is wrong. It allows the car to move but not to optimal speeds. 

The limp mode is to keep the car’s engine from sustaining damage because of an undetected problem. However, most cars don’t have the feature light up on the dash when it automatically turns on. Your car just starts slowing down to the lowest gear. 

How Much Does It Cost to Fix an O2 Sensor?

This is typically an inexpensive repair with the part costing anywhere from $20 to $100. It will cost around $200 to fix if you take your car to a mechanic once they include their labor.

Will a bad O2 sensor result in power loss?

You may feel a loss of power in your engine because of a bad O2 sensor. When it sends inaccurate information to the control module, the fuel and intake systems are affected and that leads to wrong shifting points programs into the transmission control module. 

All of that means inadequate combustion and a lack of power for acceleration. 

However, an O2 sensor doesn’t cause a lack of electrical power in the car. All of your lights, headlights, and electronics will continue to work fine.

Can a faulty O2 sensor cause a car to stall?

No, there is no reason to believe a faulty oxygen sensor will cause your vehicle to stall. These sensors relay information about the air and fuel mix and, although it can disrupt combustion, it won’t stall the car. 

The check engine light will light up when the sensor has completely failed but the car will keep working. 

However, problems will get worse over time if the sensor isn’t replaced or at least cleaned. Sometimes, a sensor may just be dirty and can be cleaned to keep everything working. A mechanic will be able to tell you which is needed.


You want your car to run its best and satisfy emission codes. That means making sure you replace your O2 sensor when it begins to fail. Trying to put it off will only cost you more in gas, repeated emission tests, and possibly more engine repairs.

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