Oxygen Sensors - Why Does My Car Have Four, And Can I Replace One Myself?

What Is An Oxygen Sensor And How Does It Work?

An oxygen sensor (also known as an O2 sensor or Lambda sensor) is a sensor designed to generate a reading based on the content of oxygen in an automotive exhaust system. The sensor itself is made of a ceramic compound with porous-shaped electrodes coated in platinum, surrounded by a protective metal shell casing. O2 sensor casings are threaded, and the entire unit is designed to screw in until it reaches a half-in, half-out position on the exhaust pipe. Modern oxygen sensors are heated to ensure they reach operating temperature quickly.

Oxygen Sensors
Lambda Sensor

An oxygen sensor generates a voltage output reading by comparing the amount of oxygen in the exhaust gas to the amount of oxygen in the outside air. A rich fuel-air mixture with a higher concentration of hydrocarbons or unburned fuel in the exhaust causes an oxygen demand. As oxygen ions are naturally drawn toward the platinum-coated electrodes inside the sensor, voltage increases. Conversely, a lean fuel-air mixture with an excessively high oxygen content over hydrocarbons will cause oxygen ions to flow away the electrodes - resulting in a lower voltage reading. As outside air conditions change, resistance may vary.

How Many Oxygen Sensors Are Required On A Typical Vehicle?
Heated Oxygen Sensor

Each exhaust pipe on a new vehicle sold in the United States is required to be equipped with a catalytic converter that converts harmful exhaust gasses into less harmful ones thanks to chemical reactions that occur inside of it. OBD II (On-Board Diagnostics II) emissions systems, on U.S. vehicles since the late 1990's, have required that each catalytic converter have one oxygen sensor mounted upstream of it, and one oxygen sensor mounted downstream of it. As a result, single-exhaust vehicles will be equipped from the factory with two oxygen sensors and dual-exhaust vehicles will be fitted with four sensors.

Two oxygen sensors per exhaust pipe allow a vehicle's engine control computer to compare the cleanliness of the exhaust before it enters the catalytic converter to the exhaust after it exits. Not only does this provide the legally required self-check to monitor if one of the sensors should fail, it allows the efficiency of the catalytic converter to be watched.

Front Oxygen Sensors
Front oxygen sensors are located in the exhaust system upstream of the catalytic converter, often deep in the engine bay. Front sensors are referred to as "Sensor 1" on scan tools.
Rear Oxygen Sensors
Rear oxygen sensors are located in the exhaust system downstream of the catalytic converter, and are referred to as "Sensor 2" on scan tools.

With this information, the vehicle's engine control unit (ECU) can offset rich air-fuel mixtures that are a natural result of cold engine starts. Some vehicles have used a secondary air pump during engine warmup, to load the exhaust system with extra oxygen from outside air - ensuring overall emissions remain clean. The newest solution to this issue eliminates the air pump, and instead uses the ECU to constantly vary the fuel-air mixture between lean and rich until the engine reaches operating temperature.

Over time, excess unburned hydrocarbons passing through the exhaust system will foul oxygen sensors and catalytic converters both, shortening their lifespan and costing the consumer a large repair bill for replacement parts.

Description Of Oxygen Sensor Location

When displaying the location of oxygen sensors, scan tools will use terms such as "Bank 1, Sensor 2". Inline engines only have one bank of cylinders, so all O2 sensors on such a vehicle will be known as Bank 1. V-shaped engines with two banks of cylinders (V6, V8, V10, V12) will register as Bank 1 or Bank 2. You may need to refer the vehicle manufacturer's service information to determine which Bank is which. Sensor 1 will always be located before the catalytic converter, and Sensor 2 will always be located after it.

How Can I Get The Longest Life Out Of My Oxygen Sensors?
New And Fouled Oxygen Sensor
Shown in picture A is a new oxygen sensor. Picture B shows an O2 sensor fouled by oil, picture C shows a sensor fouled by excess carbon, and picture D shows an O2 sensor with normal wear.

Oxygen sensors, like catalytic converters, are made of exotic materials inside which make them pricier to purchase. Extending the lifespan of these components can be done without any physical maintenance, so it makes sense to follow life-extending practices and get the most for your money. First, stay away from putting low-quality discount gasoline in your tank. You may save a little money up front, but the lower refining quality that brings bargain prices at the pump means more impurities passing through your exhaust system in the end. Over time, these will foul your emissions sensors.

Using fuel that's lower in octane than your vehicle manufacturer recommends is also harmful to your exhaust system. Octane enhances the combustion process inside your engine to ensure fuel burns more completely without waste. Take away even some of the octane your engine needs, and you've got unburned hydrocarbons going out the exhaust where they foul O2 sensors and catalytic converters.

Performing regularly scheduled tune-up maintenance such as replacing engine spark plugs and air filters is a must. When spark plugs reach the end of their life span, they too can contribute to incomplete gasoline combustion and unburned fuel in the exhaust - leading to premature demise of your emissions equipment. Vehicles with worn piston rings that burn a lot of oil will also foul oxygen sensors more often.

Replacing An Oxygen Sensor
Replacement Of An Oxygen Sensor

Replacement of an oxygen sensor is a service job within the ability of many people comfortable with performing small maintenance jobs on their vehicles. A scan tool with the ability to read engine fault codes will let you see exactly which oxygen sensor is faulty. While oxygen sensors on most vehicles are located in easy-to-reach spots on the exhaust system, some upstream sensors are tucked under the exhaust manifold - making them extremely awkward and difficult to reach. You may need to try various combinations of deep sockets, extension bars, or swivel head ratchets to reach down deep in the engine bay. A specially fitted oxygen sensor socket will provide the best grip without damaging any attached wiring during removal and installation. You'll want to let a warm vehicle cool off - burns from touching hot exhaust components tend to leave a mark.

Tools and equipment needed:
Unhooking The Wire Connector of The Faulty O2 Sensor

After confirming the location of the O2 sensor needing replacement, drive the vehicle up on ramps if you'll be working underneath it. Or, jack the vehicle up as needed and insert a jack stand or stands to support its weight. Make sure the gear selector is in park, the emergency brake is on, and wheel chocks are located behind rear wheels to ensure the vehicle stays still.

After unhooking the wire connector of the faulty O2 sensor, spray the metal threaded area with penetrating lubricant. While the exhaust system cools further, let the lubricant soak into the threads where it will loosen corrosion that normally makes the sensor to difficult to unscrew.

After you've allowed some time for the penetrant to do its job, unscrew the old sensor from the exhaust system using a fitted socket. Clean the threads inside the hole and apply anti-seize lubricant in order to create a tight seal that keeps corrosion at bay and the elements out.

Specially shaped oxygen sensor sockets
Specially shaped oxygen sensor sockets are highly recommended to provide maximum grip in tight spaces while preventing damage to sensitive components.
Anti Seize Lubricant
Before installing a new oxygen sensor, apply anti-seize lubricant on the threads in order to create a tight, corrosion-free seal.

Install the replacement O2 sensor using the specially designed socket. If you're installing the new sensor on a flange, use a new gasket before tightening. Press fit the wire connector from the vehicle onto the new sensor's connector, and you're finished. A scan tool will clear any check engine fault codes generated by the malfunctioning sensor.


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