o2 sensor

Oxygen Sensor (O2) Symptoms, Function & Location

In Engine by Magnus Sellén1 Comment

o2 sensorThe oxygen sensor, which is also known as the lambda sensor, is situated in the car’s exhaust manifold.

It measures the amount of oxygen in the gases that leave your car through the exhaust. Dr Gunter Bauman created this sensor in the 1960 decade while working for the Robert Bosch GmbH company.

The sensor became famous and was widely used in vehicles that were made after 1980. The O2 sensor acquires data from the gases leaving your engine and sends the data to the car’s central computer to analyze. The onboard computer ensures your engine is working in tip-top condition, therefore if it senses any irregularity in the exiting fumes, it notifies you via a number of ways.

However, just like any other mechanical equipment, an oxygen sensor is bound to fail sometime. If it fails, your vehicle will act abnormally, but you won’t know how to solve the problem if you don’t know how to diagnose an oxygen sensor failure. We are sure you would like to know, therefore, for your help we have created a list of symptoms and solutions that will give you valuable insight.


What is an Oxygen Sensor?

o2 sensorThe oxygen sensor is an electronic device that is installed in your car which measures the amount and concentration of the oxygen in gas or liquid. It is also known as the lambda sensor. The advanced automobile technology is capable of manufacturing the most efficient oxygen sensors. The oxygen sensor is an essential component of all vehicles that were made after the 1980s. the principal function of the oxygen sensor is to collect data on how much oxygen is available for combustion and send it to the engine management system.

The system then utilizes the data for fuel injection and other purposes. Oxygen sensor plays an important role in ensuring optimal performance of the engine and efficient fuel mileage. This sensor can also provide data about the emissions that are being produced and sends and it indicates when the engine emissions are more than the standard limits with the help of the CEL and oxygen sensor light. A bad or a failing oxygen sensor will not only affect the engine performance of your car and the efficiency of the fuel, but it will also make the emissions reading faulty. As a result, during an emission test or inspection, you can get into trouble as the emission tests would fail.

The oxygen sensor is needed by your vehicle because the oxygen intake of your vehicle can vary due to various reasons, such as the air temperature, the altitude of your car, the temperature of your engine, the load on the engine and the barometric pressure. A failed oxygen sensor will not let the computer sense the air/fuel ratio of for your engine. As a result, the readings are manipulated and the performance of the car is impacted. Oxygen sensors usually do not fail completely and their efficiency is reduced over time.


Oxygen O2 Sensor Symptoms

Cars can malfunction anytime, so you need to know when to visit the mechanic. Here are a number of early symptoms that can point towards an impending oxygen sensor failure.

Bad Oxygen (O2) SensorCheck Engine Light Illuminates

Poor Mileage

Misfiring Engine
Oil combustion(Internal oil leakage damaging sensor)

Faulty O2 Sensor

Faulty O2 sensor wirings
Repair internal leakage

Replace Oxygen Sensor

Repair faulty wirings to the oxygen sensor

Check Engine Light Illuminates

Sensors are directly connected to a car’s onboard computer. The computer receives data from the sensors and analyzes it but if it receives incorrect data or no data at all, it triggers the Check Engine Light on your dashboard. You might know by now that the Check Engine light can light up due to a number of reasons, now you know a failing oxygen sensor can be one of them.

Poor Mileage

A bad O2 sensor will distort the engine’s tuning. Not only will an incorrect amount of fuel being dumped into the system, but the air to fuel ratio will be in tatters. This will result in more fuel being used and bad mileage. If you are frequently filling up your gas tank, a failing oxygen sensor could be the reason behind it.

Misfiring Engine

A distorted fuel to air ratio will destroy the tandem inside the engine and its cylinders. This will result in more misfires, black smoke rising from the exhaust and uneven idling. The O2 sensor plays a vital role in monitoring the engine’s timing and its fuel delivery among other things, so if there is any harm to the sensor your overall engine health will deteriorate.


oxygen sensorSolutions of a bad O2 Sensor

Fortunately, sensors are small engine parts and they are easily replaced, but still, we will look at ways to solve O2 sensor issues without spending too many dollars.

Use an OBDII Scanner

We cannot stress the usage of OBDII scanners enough. It is just a vital instrument for every car owner and a great tool to diagnose car related problems, especially if they are related to sensors. That is because sensor failure is usually reported to the car’s ECU, which lights up the Check Engine light. The only way to know about the actual problem is to use an OBDII scanner. The scanner will let you know of the exact problem, which you can discuss later with your mechanic. Sometimes, the error code can be reset and the issue goes away, which means you might not have to take that trip to the mechanic.

If you want to get an OBD2 scanner to use at home, you can check out this on Amazon: OBDII Scanner

Backprobe and Voltmeter

These two things are required to test the oxygen sensor directly. Using a high impedance voltmeter and a backprobe you can test the integrity of the O2 sensor while it is still attached to the car. All you have to do is, check the wires surrounding the sensor and make sure they are in optimum condition. Once you have determined that start up the car and wait for it to heat up. Place the voltmeter and check the sensor in specific settings. Doing all this can be troublesome for many of us, so it is better to take your car to the mechanical workshop. At the mechanic’s place, you can get a clear diagnosis of the problem and learn if your sensor needs repair or replacement.


Location of the Oxygen sensor

There can be multiple oxygen sensors on your vehicle. These sensors are located in the exhaust stream of your vehicle. One of the sensors must be located before the catalytic converter installed in the exhaust manifold. Another two or one sensors would be located after the catalytic converter. This helps to monitor the performance of the catalytic converter by comparing the before and after readings. These sensors may have different locations and numbers based on the design of your vehicle.


Oxygen sensor Replacement Cost

The average replacement cost of an oxygen sensor is between $200 and $260. The part can cost between $20 and $150 while the labour cost varies between $90 and $250. The cost of oxygen sensor replacement can vary a lot depending on the brand of your vehicle and the labour involved in the repair.

From reports of different areas and vehicle types, we have calculated an average range of the costs involved in the replacement of the oxygen sensor. If you plan to replace the oxygen sensor by yourself you would generally need to buy only the parts. The price for these parts can be anywhere between $20 to as much as $150 depending on the brand of the vehicle that you have and the place you are buying it from.

If you hire a technician or a mechanic to do the job, it would be a highly variable for the overall cost, depending on how much the labor costs you for fixing the problem. It will generally cost you higher if you go to a high-end car shop than getting it fixed by an independent mechanic. If you hire a mechanic to do the job, he would charge you an additional $90 to $250 for fixing the issue apart from the costs involved in buying parts. Due to such a high variation in the prices, it is a good idea to do some market research and get multiple quotes from the mechanics. You may also ask people in your local area who have recently got it replaced or repaired.


Repairing/Replacing oxygen sensor

The oxygen sensor is a small piece of equipment situated in a very sensitive area, meaning it can get damaged very easily. The wires surrounding the sensor are also prone to damage, so it is not unnatural to face oxygen sensor failure. Experts even recommend changing the O2 sensor after a few thousand miles just like you change the oil in your car. However, sensors can last longer than oil, way longer, so you would probably have to empty your pockets after every 50,000 – 70-000 miles. This also means the sensor is not invincible, it is a mechanical part, which can survive for so long.

Make sure you are visiting your mechanic for regular service and checkups so that this can be avoided. However, if you have no other choice than to change and replace your car’s oxygen sensor, it is wise to consider the many price alternatives.

Ones the oxygen sensor reaches its end life, the best option is to replace it in order to avoid continued trouble. In order to replace the oxygen sensor, here is what you need to do:

• Locate the oxygen sensor that you need to replace
• Consult to a service manual of your vehicle to see where they are located in your vehicle design
• It should look like a spark plug sticking out of the exhaust pipe
• Disconnect the electrical connectors from the oxygen sensor
• Unscrew the oxygen sensor from the exhaust stream
• Match the old one and the new one
• Replace and install the new one from where you removed the old one
• Connect the electrical connectors that you removed
• Turn on the ignition without starting the vehicle
• Run the diagnostic scanner to clear the code
• Start the vehicle and observe the changes


In-depth Diagnosis of a Failing Oxygen Sensor

Your vehicle can experience a number of changes if the oxygen sensor goes bad, almost all of them would be negative changes. The check engine light is an important indicator for you to know that there is something wrong with your vehicle that needs to be taken care of, and it will light up in case of a bad oxygen sensor. Another sign that would indicate a failing oxygen sensor is a message on the computer of your car which says that the heater circuit has malfunctioned. These two signs together would mean that it could possibly be a failing oxygen sensor. These signs can also be due to high emissions due to the bad performance of your car because of some other reason. Usually, it could be complicated to know this by online checking these things.

Checking, testing and diagnosing for a bad oxygen sensor can be tricky and time-consuming. Therefore, it is better to simply take it to the expert mechanic and get it all done if you don’t have enough knowledge about it. If you are familiar with these things and have some prior knowledge, you can go about doing it easily. Here is a step by step guide that you can follow to carry out diagnostic tests for the bad oxygen sensor.

• Use a 10 megaohm digital meter to prevent any damage
• Locate the oxygen sensor using the service manual
• Use the service manual to identify the bad sensor with the help of diagnostic trouble codes (DTC)
• Choose the one among a number of sensors that you want to test
• Locate the Bank 1 and bank 2 with the help of the service manual
• Locate the signal wire with the help of the manual
• Warm up your car with a 20 minutes’ drive
• Turn off the engine
• Lift the vehicle with jack stands if required
• Get the voltmeter and set it to DC millivolt scale
• Avoid burning yourself and use protective wears
• Connect the red cable of the meter to the signal wire
• Connect the black with the engine ground or sensor’s ground wire if given
• Start your engine
• Read the voltage signals from the sensor on your voltmeter. It should be between 0.1 to 0.9 V
• If the values are over or under this range, this may be due to a bad sensor or engine performance.
• If improper values, Test the Oxygen Sensor Response to a Lean Fuel Condition.
• Then Test the Oxygen Sensor Response to a Rich Fuel Condition

Hello I'm Magnus, the owner and the writer of this website. I have been working with cars since I was 16 and I'm specialized with in-depth Automotive diagnostics. Also been driving drifting for the last 6 years. I'm here to give you answers to all your automotive questions and I hope that you enjoy our content.

  1. I an looking for oxygen 02 sensor for mazda cx 7 2012 model 2.5 engine

  2. Due to a very high demand and high ammount of comments, you have to wait for some time for your car questions to get answered. If you want to get fast answers from a certified master technician you can ask your questions here:
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