Modern cars have many different sensors onboard, including two or more oxygen sensors. The oxygen sensors are located in the vehicle’s exhaust system, where they’re often subject to contamination and high operating temperatures.
Because the sensors live such a tough life, it’s not uncommon for them to fail prematurely, resulting in one or more noticeable symptoms. Let’s cover the most common symptoms of a bad oxygen sensor.
What Does an O2 Sensor Do?
Oxygen (O2) sensors measure the amount of oxygen (or the concentration of combustibles) in the exhaust gases leaving the engine. O2 sensors were first introduced in production vehicles in the 1970s and became commonplace by the 1980s.
So what does the O2 sensor do in your car? It measures the oxygen content in the exhaust gases leaving the engine. The amount of oxygen shows how much of the fuel got burned.
And where is the oxygen sensor located? Modern vehicles have both an upstream and a downstream O2 sensor. Upstream O2 sensors are located before the catalytic converter, while downstream sensors are located after the converter.
The engine computer, which is often referred to as the powertrain control module (PCM), uses data from the upstream O2 sensor to regulate the engine’s fuel mixture. Meanwhile, the PCM primarily uses the signal from the downstream O2 sensor to monitor the catalytic converter’s health. In some cases, the PCM may also use the signal from the downstream sensor to tweak the fuel mixture.
Most older vehicles have O2 sensors with one or two wires. But late-model vehicles have four-wire oxygen sensors—two of the wires are for the sensor’s signal circuit, and two are for the sensor’s heater circuit. The PCM turns on the heater circuit when the vehicle is first started to quickly bring the sensor up to operating temperature.
There are two primary types of O2 sensors in use today: conventional zirconia and wide-band. A conventional O2 sensor can only react as a rich-lean indicator to an air-fuel mixture within a certain range. On the other hand, a wide-band O2 sensor can detect the exact mixture over a much wider range for more precise system monitoring.
Watch this video to understand how oxygen sensors work:
Bad Oxygen Sensor Symptoms
You might be wondering: What are the symptoms of a bad O2 sensor? What happens when your O2 sensor is bad? The following signs are common indicators that one of your car’s O2 sensors is no longer doing its job.
Note: Many other problems can mimic a bad O2 sensor. You’ll want to perform a thorough diagnosis before condemning the sensor itself.
Illuminated Check Engine Light
One of the most common signs of a bad O2 sensor is an illuminated check engine light. The PCM monitors the operation of the O2 sensors and their heater circuits. If the device detects a problem with any of the O2 sensors, it turns on the check engine light and stores a corresponding diagnostic trouble code (DTC) in its memory.
Engine Performance Problems
The PCM uses the signals from the O2 sensors when calculating the engine’s air-fuel mixture. As such, a faulty O2 sensor can result in the PCM giving the engine the incorrect amount of fuel, causing issues, such as misfiring, rough running, and stalling.
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I have a 2008 Ford Fusion SEL, 3.0 V6. Recently, it threw a fault code regarding two O2 Sensors. Prior to this, I was experiencing no unusual idle, acceleration problems. I drove it for another week until I could find the time to replace them. I did not experience any noticeable difference. After that week, I cranked it to leave for work, and it began idling very roughly for a minute or two and then stalled. When I turned it back on, it had a new fault code regarding the throttle body. I parked it for another week, during which I removed the throttle body to clean it and check for any damage. I disconnected the battery prior to removal and used [a] Throttle Body cleaner. It wasn’t super dirty like other photos I have seen, but I still cleaned and inspected it. I believe, in theory, it should operate as normal. Provided the TPS isn’t malfunctioning. I checked and cleaned all of the connectors and MAFS as well, and they appear in good shape. After doing some research online, I discovered that the TPS failing can be a common issue with this specific vehicle and the years produced. The TPS and throttle body are, unfortunately, a one-piece part that is rather expensive for OEM or comparable. I have not yet reinstalled my throttle body out of caution that I will have to take it back off and purchase a new one anyway.
Increased Fuel Consumption
An O2 sensor that is sluggish or has failed altogether can lead to an overly rich air-fuel mixture and increased fuel consumption. The unburned fuel can also cause the catalytic converter to overheat and fail.
Exhaust Fumes Smell Like Gasoline
A dirty O2 sensor can cause excess fuel to enter the combustion chamber. When this happens, the unburnt fuel exits through the exhaust, causing the exhaust gases to smell strongly of gasoline.
How Many O2 Sensors are in a Car?
Exactly how many O2 sensors are in a car will depend on factors, such as the year, make, model, and engine size. Most late-model vehicles have at least two oxygen sensors—an upstream and a downstream—for each of the engine’s cylinder banks. Engines with a ‘V’ or flat configuration have two cylinder banks, whereas engines with an inline or flat configuration have just one bank.
FAQ
Sometimes, O2 sensors simply fail due to time and miles. In other cases, an O2 sensor may go bad due to contamination from substances, such as oil ash and coolant. Environmental factors (e.g., water and road salt) can also cause an O2 sensor to fail.
If you choose to have a professional replace your car’s oxygen sensor, you can usually expect to pay somewhere between $200 and $500 to get the job done. Of course, the exact cost will depend on various factors, such as the year, make, and model of your vehicle.
You can save money by replacing the oxygen sensor yourself if you have the tools and the know-how. CarParts.com has a wide variety of replacement oxygen sensors available for various makes and models.
O2 sensor replacement is generally a fairly straightforward, DIY-friendly task. Because O2 sensors are located in the exhaust, they often succumb to rust and corrosion, making them difficult to break loose.
As such, it’s a good idea to apply penetrating oil to the threaded portion of the O2 sensor before removal. Also, it’s highly recommended that you consult a repair manual or repair database before attempting to replace the sensor.
If your car is suffering from a bad O2 sensor, you should address the issue as soon as possible. Depending on how the sensor fails, the issue can lead to additional problems, such as catalytic converter damage.
In most cases, it isn’t necessary to replace all of your car’s O2 sensors unless the sensors are contaminated. For example, if the engine has suffered from a failed head gasket, resulting in coolant entering the exhaust stream, all of the O2 sensors may require replacement due to contamination.
What Happens if an Oxygen Sensor Is Dirty?
A faulty O2 sensor can interfere with several engine functions, such as engine timing, combustion intervals, and air-fuel ratio. This can cause misfires, stalling, poor acceleration, and other engine issues.
Will a Dirty Oxygen Sensor Throw a Code?
Yes, a faulty O2 sensor is likely to set off a code. One example is the P0138 code, which stands for “O2 Sensor Circuit High Voltage (Bank 1, Sensor 2).”
While these codes can certainly help you diagnose the issue, there are cases where they have several possible causes.
For example, the P0171 code, or “System Too Lean (Bank 1), is triggered when the oxygen sensor detects a lean mixture. Possible causes include a worn-out O2 sensor, clogged fuel injectors, or a faulty fuel pump.
Oxygen Sensor Replacement Tips
The process of replacing an oxygen sensor can differ depending on your vehicle’s year, make, and model. In general, here are some handy tips if you need to replace your vehicle’s oxygen sensor:
Tools Needed
Have a hydraulic jack, some jack stands, and a torque wrench on hand for this project. Also grab a 22mm wrench or whatever wrench fits your oxygen sensors.
Preparation
Warm up your vehicle by starting the engine, but make sure to wait long enough for the hot portions to cool down before starting any work so that you don’t burn yourself. Remember to turn off the ignition and set the parking brake. Disconnect the battery and raise the vehicle onto jack stands so that everything is stable when you work.
Replacement & Reassembly
You’ll typically find the oxygen sensors on the catalytic converter. You can remove the old, faulty sensors by pressing the retaining tab and pulling the terminal. Don’t forget to also remove the protective covering from your replacement oxygen sensors before installing them.
After you remove and replace the inoperative oxygen sensors, make sure to secure all the wiring and reconnect the battery. We’ve included more detailed instructions for these steps in the video later in this article.
Get a Replacement Oxygen Sensor that Fits Your Car
Driving with a faulty O2 sensor leads to various head-scratching issues, such as rough idling, poor acceleration, misfires, and more. They can make it risky to drive your vehicle, and fixing them usually takes a toll on the wallet. Thankfully, you can prevent these issues by shopping for a replacement sensor with the help of CarParts.com.
You don’t even have to leave your home to browse replacement parts. Simply go to CarParts.com, and use our vehicle selector to get a filtered list of the O2 sensors compatible with your ride. We’ve partnered with trusted industry manufacturers, so you know our replacement parts are top-notch. Plus, our warehouses are strategically located across the country to guarantee fast shipping.
So don’t wait until your O2 sensor stops working before you replace it. Shop for high-quality O2 sensors now.
Check out these tutorial videos on how to replace your oxygen sensors:
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