5 Symptoms of a Bad Oxygen Sensor (And Replacement Cost)

The oxygen sensor , also known as an O2 sensor or lambda sensor, is located inside of the exhaust manifold of a vehicle. Its purpose is to keep track of the level of oxygen which remains unburned in the exhaust system while exhaust is leaving the engine.

The oxygen sensor sends a signal to the ECU indicating how much oxygen was found in the exhaust. From there, the ECU adjusts its air fuel ratio to optimize combustion given the current situation, and the cycle repeats. This is called closed loop mode.

Closed loop is typically used during cruising and light load conditions. When you step on the gas and ask for full power, most vehicles ignore input from the oxygen sensor, which is called open loop mode. This will give you worse fuel economy and a richer air fuel ratio while the ECU uses preprogrammed values to determine how much fuel to inject. 

Bad O2 Sensor Symptoms

The oxygen sensor of the vehicle needs to be in working order so that the air fuel ratio can be balanced. If the oxygen sensor were to become damaged or malfunction, the vehicle will run in open loop all the time, which is very inefficient and not good for the environment.

Below are the top symptoms you will experience when the oxygen sensor goes bad in your vehicle.

1) Poor Vehicle Performance

car won't accelerate

The vehicle may feel sluggish, idle rough, stumble, or hesitate. An air fuel ratio that is excessively rich for too long may cause misfires, foul spark plugs, or even gum up the catalytic converter.

2) Failed Smog Test

black smoke from exhaust

If you go to get an emissions test on your vehicle and it fails, then it’s likely because your car is emitting too much pollution into the atmosphere in the form of carbon monoxide, hydrocarbons, and smoke.

These harmful emissions will increase when the air fuel ratio is not where it should be, and this is often caused by faulty oxygen sensors.

3) Bad Smell

car smells

Do you notice a rotten egg-like smell coming out of your exhaust? This is likely an early warning sign that something is wrong with your oxygen sensor.

If left untreated, this rotten egg smell may even spread to the inside of your car which will make it a very unpleasant experience while driving. You’d be better off taking care of it right away.

This smell could also indicate a problem with your catalytic converter

4) Check Engine Light is On

check engine light

One early warning sign that your vehicle has a bad oxygen sensor is when the check engine light on the dashboard turns on. Although this could happen for a variety of reasons, the simplest reason is because of a bad oxygen sensor.

Regardless, you should always bring your car to a mechanic as soon as you see the engine light turn on. It could mean any number of problems with your engine and you certainly don’t want to risk letting them go for too long.

5) Bad Fuel Mileage

Smart car MPG

When you have a bad oxygen sensor, the mixture of fuel and air will likely be too rich. In other words, a lot more fuel will be consumed in the internal combustion process.

This will mean that your miles-per-gallon will decrease because your car will consume more gas just to drive fewer miles. So, if you notice that you have to fill up your gas tank more than usual, that is likely caused by a bad oxygen sensor.


Oxygen Sensor Replacement Cost

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O2 sensor replacement cost

To replace an oxygen sensor, the average cost will likely end up somewhere in the range of $200 to $450 if you bring your vehicle to a mechanic. The price will heavily depend on the make and model of your vehicle as well as the shop you bring your vehicle to.

The cost of parts can vary quite a bit so you’re looking at anywhere from $50 to $300 in most cases although it can be hundreds more for certain vehicles. The labor cost will likely be between $150 and $300.

 There may also be additional fees and sales taxes added onto these prices as well.

What Does a Faulty Oxygen Sensor Look Like?

There is no visual way to determine if an oxygen sensor has gone bad. The main way to know if you have a faulty oxygen sensor is if the check engine light in the vehicle comes on and you experience one or more of the symptoms above.

How Can You Tell Which O2 Sensor Is Bad?

To tell which O2 sensor is bad, you can use an OBD2 code reader, a digital multimeter, or an automotive diagnostic scanner with live data capabilities.

When using an OBD2 scanner, an O2 sensor related trouble code will show the sensor location by sensor number (1, 2, 3, or 4) and cylinder bank (1 or 2). Once you identify the sensor that needs to be replaced, you can replace it with a new one.

Will a Faulty O2 Sensor Cause Sputtering?

Yes, a faulty oxygen sensor can cause sputtering in a car’s engine. This is because a failing sensor can disrupt the fuel amount in the engine, leading to an incorrect fuel-to-air ratio. This can cause the engine to sputter or hesitate during acceleration.

If you experience sputtering in your engine, it’s recommended to have your O2 sensor checked by a mechanic to determine if it needs to be replaced.

Can a Bad O2 Sensor Cause a Gas Smell?

Yes, a bad O2 sensor can cause a gas smell in a car. Like above, a failing sensor can cause an improper fuel-to-air ratio, causing the engine to run rich and emit a gasoline smell. If you notice a gas smell in your car and one or more of the additional symptoms above, it’s a good idea to get it checked out by a mechanic.

Can I Replace My Own Oxygen Sensor?

Replacing an oxygen sensor at home is often fairly straightforward. Some vehicles have multiple oxygen sensors, so make sure you buy and replace the right one.

You will need an O2 sensor socket to remove the old sensor and install the new one. Here are three great options for getting the job done:

A repair manual will help you figure out what steps you need to take to replace your oxygen sensor, but this information can often be found online if you search for your specific year, make, and model.


7 thoughts on “5 Symptoms of a Bad Oxygen Sensor (And Replacement Cost)”

    • The sensor could be fouled from a rich air fuel mixture, or there could be a wiring issue. Sometimes cars will throw O2 sensor codes for other issues as well; it may not be a problem with the O2 sensor itself.


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