How to Detect and Replace a Faulty Oxygen Sensor

January 27, 2012

The oxygen sensor is a vital part of a gasoline engine's sensor array. The sensor array consists of the electronic measuring tools your engine's computer uses to properly regulate the operation of your vehicle's engine and keep it running smoothly and safely. This includes calculating the proper air/fuel mixture for the conditions under which your motor is running. As sensors age they become less and less accurate, which can result in excess consumption of fuel.

Faulty Oxygen Sensors
The main way that you will know if you have a faulty oxygen sensor is that the check engine light in the vehicle will come on. The vehicle is not likely to be experiencing any major engine issues, sounds or anything else that would clue you in on the issue. The oxygen sensor is governed by an OBD (on board diagnostic) system that reports when the sensor has failed. When your check engine light comes on, you can't tell what the problem is. You need to hook up the engine computer to a reader (most shops have them on hand) to see what the diagnostic codes are. These will inform you if you have an oxygen sensor problem.

Why Oxygen Sensors Fail
There is a lot of heat applied to oxygen sensors. Over time, it can have a negative effect. Most oxygen sensors are rated for 100,000 miles. After this point, they have a chance of failing. An oxygen sensor may or may not need to be replaced when an OBD code signals sensor issues. Rather than quickly paying for a replacement, talk to a local shop about doing more in depth exhaust diagnostics to figure out if it really time to retire an oxygen sensor that is not performing correctly.

How to Replace a Faulty Oxygen Sensor

The oxygen sensor is relatively easy to locate and replace. It can be done at home by the amateur mechanic.

Tools and Materials

  • Clean rags
  • A large adjustable wrench or set of crescent wrenches
  • 2 good-sized blocks of wood
  • A flashlight
  • A car jack
  • Jack stands to support the car
  • A new oxygen sensor
  • Latex or nitrile gloves (optional)

It might also be useful to have a guide on hand for your specific vehicle, or check out the process in an online forum specific to your make and model.

Prepare Your Work Area
First, park the vehicle on a level surface and set the brake. Put the wood blocks behind and in front of one of the rear tires. This is to ensure that the vehicle doesn't move. If you've recently driven the car there's a good chance that the exhaust system will be extremely hot. Let it cool for awhile before touching it. Raise each side of the car using the jack until the tire is no longer touching the ground. Lower the car onto the stand, being careful to use one of the frame contact points identified in your owner's manual. Other places on the vehicle are not designed to support weight and will be damaged if you use them to lift or support the car. Once the car is situated on the stand make sure that it is being supported in a stable manner.

Locate and Remove the Sensor(s)
While some vehicles are equipped with only one oxygen sensor, other cars use two. On engines using only one sensor, it is typically located on or very near the exhaust manifold (the part of the exhaust that attaches to your car's motor). On vehicles with two sensors, the second sensor will be located downstream from the car's catalytic converter. Simply follow the car's exhaust system from the manifold back towards the muffler to locate the sensor. Remove and replace the sensors one at a time, taking care to make sure you're installing the correct sensor in each location. Take a look and the new oxygen sensor so that you have a good idea of exactly what it looks like. The sensor wire should unplug using a simple clip. Once the wire is removed, use a crescent wrench to loosen the bolt attaching the sensor to the exhaust system and remove it. Replace the sensor and reconnect it, taking care to make sure the wire's clip clicks into place. Take care to route the wire in its original location so that the wire is away from the hot exhaust pipe.

Finish Up
Once the sensors are replaced, reverse the process of step one to complete the job.

Why Your Oxygen Sensor is Failing the Emissions Test

The oxygen sensor measures the level of fuel and gases such as oxygen and nitrogen that leave the engine cylinders. The sensing element is a zirconium ceramic bulb coated on both sides with a thin coat of platinum. The outside of the bulb comes in contact with the hot exhaust gases and the inside of the bulb is vented to the outside atmosphere through the sensor body or wiring. The perfect output voltage of the sensor should be 0.45 volts. If the sensor detects too much fuel or gases, the ECU reduces the flow of fuel into the engine, and if the sensor detects too little fuel, the ECU increases the amount of fuel entering the engine.

The Oxygen Sensor and Your Car's Emissions Test
An emissions test, also known as a smog check, checks the pollutants emitted by a car's engine. If the oxygen sensor isn't working properly, the exhaust gasses can't be properly regulated. When the exhaust emissions measured by the emission analyzer are too high, the car fails the emissions test. One of the prime reasons for this could be a faulty oxygen sensor. If you find that your car's gas mileage is adversely affected or the car surges or hesitates while driving, the reason could be a defective oxygen sensor. A defective sensor can damage the car's catalytic converter. If the car fails the emissions test due to high carbon monoxide, you should check the oxygen sensor.

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