The 5 Telltale Signs Your O2 Sensor Has Gone Bad


An O2 sensor is a sensor in your car that measures oxygen levels in your exhaust. O2 is the chemical compound for oxygen as oxygen molecules always travel in pairs of two, sharing a valence electron. Hence the name O2 sensor. 

The O2 sensor is a part of the vehicle that most people haven’t even heard of. Despite its lack of fame, the O2 sensor is vital to the efficiency of the vehicle as well as the health of the emissions the car emits. Cars need fuel to run so we fill them up with gasoline, but cars also need oxygen to ignite that fuel! 

Moreover, cars need the proper ratio of gasoline to oxygen to run effectively. On average, cars need 14 grams of oxygen for every 1 gram of fuel. Cars use their O2 sensors to make sure they are getting the proper ratio of oxygen to fuel and this helps them run more efficiently. 

When an O2 sensor goes bad, a few things will happen:

  1. The check engine light will come on
  2. Your gas mileage goes way down
  3. Rotten egg smell
  4. Rough Idling
  5. Engine will misfire/loss of power

We’ll get into the details of these five things in just a bit, but first it’s important to understand how an O2 sensor works to fully comprehend why it results in engine misfires and poor gas mileage. 

So How Does An O2 Sensor Work?

The O2 sensor is a tiny device about the size of a spark plug which is found within the exhaust system of your vehicle. Here, it measures the amount of oxygen that is in the exhaust. The O2 sensor measures how much oxygen is in the exhaust produced by the car and compares it to the oxygen levels of the air. 

This data is then sent to your car’s computer system which processes it. From there the computer system will make changes to the ratio, helping your car run more effectively i.e. better gas mileage and less emissions. Today’s computerized car engines utilize sensors throughout the vehicle to enhance and regulate engine performance. The O2 sensor is just one example of this. 

As your car’s computer interprets the information gathered by the O2 sensor it changes the mixture of the fuel supplied to the engine. This is a constant loop where the O2 sensor reads the oxygen levels of the exhaust, sends the information to the computer and the computer dictates the fuel mixture. So the fuel mixture is constantly changing from lean to rich levels.  

When you have an O2 sensor that is broken or not relaying data properly the computer will opt to send out a highly rich blend of fuel to the injectors. This is the same mix that is sent when you first start your car. When this happens the fuel supplied will constantly be rich which not only increases the total fuel consumption but the amount of carbon that your car is spitting out into the atmosphere. 

What Are The 5 Signs Of A Bad O2 Sensor

  1. Your check engine light is on. A malfunctioning or broken O2 sensor is the number one reason that check engine lights come on across the world. Your car is alerting you to a problem it is experiencing and some are more severe than others. Always be sure to diagnose a check engine light right away either with your mechanic or at home using the proper tools. 
  2. You are burning through fuel at an unusually high rate. When your O2 sensor breaks, your car begins burning gasoline like crazy because it has no idea how much oxygen the car is getting and will automatically convert to giving the richest fuel supply. This is bad both for your wallet and for the environment as your car will suck down gas at an abnormally high rate and put out a ton of extra emissions.
  3. Rotten egg smell. When your O2 sensor malfunctions the vehicle will again begin injecting greater volumes of fuel into the car. This extra fuel can cause your car to emit a sulfuric or rotten egg smell. You may also see a black smoke coming from your exhaust. These types of bad emissions can be caused by other system issues, but smoke coming out of the tailpipe is never a good thing. Get the car looked at right away. 
  4. Rough idling. Engine timing, combustion intervals, and air to fuel ratios are all controlled by the O2 sensor. A bad sensor can disrupt the way the engine runs which can lead to a feeling of roughness when idling or operating the car in general.
  5. Engine misfires. Because the O2 sensor dictates the lean or rich quality of the fuel that your engine receives, a broken O2 sensor can lead to your engine misfiring. 

Why Do O2 Sensors Go Bad?

O2 sensors degrade for a variety of reasons from being contaminated over the years with harsh fuel additives to oil leaks. Generally O2 sensors have a maximum lifespan of about 100,000 miles at which point they need to be replaced. 

Can You Drive Your Car With A Bad O2 Sensor?

Yes, you can still drive a car that has a bad O2 sensor but for the sake of your wallet, your car and the environment you really shouldn’t. When you drive a vehicle with a malfunctioning O2 sensor what happens is the vehicle provides a very rich fuel blend to the engine because it is getting no reading from the O2 sensor telling it what blend to give. 

Not having a properly functioning O2 sensor will lead to engine misfires and rough and sluggish driving along with the engine stalls and far more than normal levels of pollution. If you have to drive your car, you can but it is best to get the exhaust system looked at as soon as possible and not go more than a few days of driving the car in this condition.

If left neglected it can lead to other problems in the engine and exhaust systems which are far more costly to repair including the catalytic converter. 

Can A Bad O2 Sensor Cause Loss Of Power?

Yes, loss of power can be a symptom of a bad O2 sensor typically caused by engine misfires. When the ratio of oxygen to fuel is thrown off, your engine will struggle to function optimally and you may feel like your car is sluggish and not driving well. 

How Much Does It Cost To Replace An O2 Sensor

The cost of replacing an O2 sensor is generally on the lower end of the average car repair sitting around $200 however the cost of the repair will depend on what the issues with the O2 sensor are. There are several components that fall under the O2 category from wires, hoses, engine ground corrosion, the catalytic converter, engine control module and meta tabs. 

On average the cost of fixing these parts is as follows:

Broken Wire: $100- $200 with an average cost of $145 including both parts and labor. 

Exhaust Leak : $100- $200 with an average cost of $170 including both parts and labor

Replace oxygen sensor: $200- $300 with an average cost of $225 including both parts and labor

Replace the catalytic Converter: $400- $2500 with an average cost of $1250 including both parts and labor

How Long Does It Take To Have An O2 Sensor Replaced

The O2 sensor is fast and simple to replace. The entire replacement generally won’t take longer than 10 minutes to complete. When compiled with resetting the computer, the entire repair should take no longer than half an hour. This makes the labor costs on this repair very low. The part is also inexpensive making it one of the better repairs you will eventually need to make on a car. 

As O2 sensors age, they degrade thanks to outside contaminants. Typically a fuel sensor that was manufactured any time in the last 15 years will last from 60,000 to 100,000 miles before needing to be replaced. When you see the symptoms of an illuminated check engine light, sulfur smell, black smokey exhaust, poor gas mileage, rough idling or engine misfires, be sure to take your car to a trusted mechanic immediately! Luckily, this repair won’t run you more than $200 to fix. 

If you are concerned for the health of your vehicle in this area, call a mechanic and schedule an exhaust and emission system inspection. 

If you have an extended warranty through PMC check out your policy to see if you are covered for O2 sensor replacement. You could pay next to nothing for your repairs with your policy through Protect My Car. 

Protect My Car provides consumers with extended auto warranty plans that have real coverage for vehicles that are no longer covered by their manufacturer’s warranty. Whether your vehicle was purchased new or used, if your manufacturer’s warranty is about to expire, or has already expired, an extended auto warranty plan can save you thousands of dollars in repair bills. Since the majority of vehicle repairs happen 3-5 years after the original purchase date, which many times is outside of the manufacturer’s warranty coverage period, leaving you responsible for paying the full repair bill. However, when you purchase a policy from Protect My Car, you could pay as little as $100.00 for your major repairs. That’s a lot of savings!


About Protect My Car

Protect My Car is an extended auto warranty company. Our goal here at Protect My Car is to eliminate your worry of being financially responsible for an expensive mechanical breakdown. With our extended auto warranty, you don’t have to worry about being fully burdened with the cost of a covered repair.