About Oxygen Sensors
Jared Fenton

Difficulty Level: 1
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     In this article, we will discuss oxygen sensors, what they do and how to test oxygen sensors. I will try to explain a little about them and answer a few questions. Keep in mind that this article is not vehicle specific, but applies to all cars in general. As always, if you are in doubt about something, consult your owner’s manual or repair guide for vehicle specific information.

When should I replace the oxygen sensor?

     Usually, oxygen sensors require replacement around 60 to 100K miles. You should check the owner’s manual or repair guide for the recommended mileage for your car. Most modern cars have a service light or gauge that lights up when the preset mileage has been reached. This light is usually triggered by a few different things, such as a mechanical mileage counter or by counting a certain amount of turns of the key in the ignition. In these cases, the light is merely a reminder to service the sensor. Be sure not to confuse a service reminder light with an actually failure light. A failure light is a light that only illuminates when the sensor actually fails. Check the owner’s manual or repair guide if you are in any doubt as to whether or not the light is a failure light or service light.  It is not always necessary to replace the sensor at the factory recommended mileage however. These are usually just a service recommendation. Naturally, parts do wear out and fail over time, so it’s always a good idea to practice preventative maintenance on your car.

What are the symptoms of a bad oxygen sensor?

     The biggest indicator of a faulty oxygen sensor is a noticeable decrease in fuel economy, along with a rich mixture. Now, this does not automatically indicate that the sensor has failed. Be sure to check all vacuum hoses for leaks as well as the ignition system, check the plugs, (are they fouled?) check the rotor, distributor cap, points, spark plug leads, and condenser (check all of these where applicable) Vacuum leaks and ignition problems are notorious for causing fuel economy problems. It’s a good idea to inspect and re-new the vacuum hoses every couple of years anyway. Other symptoms of a faulty sensor are a loss of power, (particularly when accelerating from a stand-still), overheating, and spark plug fouling, both of these are due to an increased rich or lean running condition. 

     Most modern cars nowadays come with sort of a built in troubleshooting guide that can help you diagnose if the sensor is bad. There is usually a port or connector where you can plug in a computer and extract codes from the fuel injection computer. In the case of most new cars, when an error occurs, it generates a code and stores it. Using the computer, you can extract the code from the fuel injection computer. This is invaluable in determining not only a bad oxygen sensor, but all sorts of other problems as well. In many cases, this can tell you if the sensor is bad, however if it does not, keep reading, the steps below will go over how to test the sensor.

What can damage the oxygen sensor?

     Sometimes, but not always, certain home or shop repairs can possibly damage an oxygen sensor. Usually, the sensor is placed in the exhaust manifold very near the bottom of the engine. It is in this area that you have components such as oil pans, valve covers, and other panels and covers that are usually sealed with a silicone-based sealer such as RTV. Some of these sealers contain fumes that will damage the sensor. If you are working in the area of the sensor, be sure to use a silicone-based sealer that is labeled “Oxygen Sensor Safe” You can find this sealer at any auto parts store. Other things that can destroy a sensor are anti-freeze/coolant, using leaded fuel (even though it is hard to find these days), as well as a sustained rich running engine. (This is usually the case when the oxygen sensor fails) When the engine is running rich for an extended amount of time, carbon will begin to build up on the inlets for the sensor, clog it, and eventually burn it out.

Is it safe to test an oxygen sensor?

     As long as you are merely testing the voltage output, it is usually safe to test the sensor. You do not want to apply any sort of voltage to the sensor, this can fry the sensor. Also, you do not want to check resistance between terminals as checking resistance means that you are sending voltage into a circuit, and measuring the amount returning. This can fry the sensor as well.

How does this thing work?

     Oxygen sensors are essentially chemical generators. They work by constantly measuring the oxygen content inside the exhaust manifold and comparing it to the air outside the engine. If this comparison shows little or no oxygen in the exhaust manifold, a voltage is generated. This voltage is then sent to the fuel injection computer, where it is received and based on the voltage, the fuel injection computer makes the necessary adjustments to change the overall fuel-air mixture of the engine. When the oxygen sensor measures the correct mixture, the voltage drops and it sends the appropriate signal to the computer to stop adjusting the mixture. This is a non-stop exchange of signals between the sensor and the computer, and it is constantly making adjustments depending on the needs of the engine. When the sensor fails, it stops sending voltage to the fuel injection computer, and usually the computer interprets this as “ok, we aren’t getting a signal, so we better enrich this engine as much as we can”, hence a rich running engine.

Oxygen sensors usually put out a very small voltage when measuring oxygen content. Typically, they only put out from 0 to 1.1 volts max. 

     All internal combustion engines need a proper air-fuel ratio in order to run correctly. Gasoline engines need to run a ratio of 14.7 parts of air to one part of fuel. (Air cooled motors typically need to be slightly richer).

     When an engine has more fuel than it needs, all the oxygen in the cylinder head is consumed and the resulting exhaust gases contain almost no oxygen. This usually causes the sensor to generate a voltage of greater than 0.45 volts. If the engine is running too lean, all the fuel is burned, and the extra oxygen remaining flows out into the exhaust manifold. The oxygen sensor detects the extra oxygen and the voltage signal will drop to below 0.45 volts.. 0.45 volts is usually the voltage where the car is neither running rich nor lean, this is when the car’s mixture is usually perfect.

     Oxygen sensors usually generate an output voltage between 0.2 to 0.7 volts. However, Oxygen sensors doe not begin to generate full output voltage until it reaches about 600 degrees F. Some oxygen sensors have a built-in heating element that lets the sensor heat up quicker.  Keep in mind that most oxygen sensors work in an open loop until they have reached a certain temperature. What this means is that until it has heated up, the car will not use the oxygen sensor to measure the mixture, but rather use all the other sensors on the car. The sensor will generate a constant 0.45 volts until it has heated up.  Once the sensor reaches a certain temperature, it becomes a closed loop and the sensor starts to generate variable voltage.

How do I test the sensor?

     The first step is to let the car warm up to operating temperature. You will need a high-impedance DC voltmeter to measure the output voltage. It’s a good idea to use a high quality or digital voltmeter. Analog voltmeters usually are not sensitive enough to register the small voltages generated by the oxygen sensor.

       The first step is to get the engine warmed up to operating temperature. This insures that the oxygen sensor will generate voltage. Now, attach the positive lead of the voltmeter to the oxygen sensor output wire.

     This wire should remain connected to the harness going to the computer, so you may find it necessary to use a jumper or trim back the insulation so you can attach the leads. Connect the negative lead to a good engine ground, such as the engine block, or any bare metal on the vehicle’s chassis. Now, set the voltmeter to look for 1 volt DC.  When you turn the key on, do not start the engine. You should see a change in voltage on the meter in most late model cars. If not, check the connections.

     Now start the engine.  In the case of single wire sensors, you should run the engine above 2000 rpm for a few minutes to heat up the O2 sensor and try to get into closed loop. The sensor showing several cross counts per second indicates closed loop operation. It may help to rev the engine between idle and about 3000 rpm several times. The computer will recognize the sensor as hot and active once there are several cross counts.

     You are looking for voltage to go above and below 0.45 volts. If you see less than 0.2 and more than 0.7 volts and the value changes rapidly, you are through, your sensor is good. If not, is it steady high, near 0.45 or steady low?  If the voltage is near the middle, you may not be hot yet. Run the engine above 2000 rpm again. If the voltage is steady high, create a vacuum leak. Try pulling the PCV valve out of its hose    or remove the oil filler cap and letting air enter. You can also use the power brake vacuum supply hose. If this drives the voltage to 0.2 to 0.3 or less and you can control it at will by opening and closing the vacuum leak, the sensor is usually good. If you are not able to make a change either way, stop the engine, unhook the sensor wire from the computer harness, and reattach your voltmeter to the sensor output wire. Repeat the rich and lean steps. If you can't get the sensor voltage to change, and you have a good sensor and ground connection, try heating it once more. Repeat the rich and lean steps. If still no voltage or fixed voltage, you have a bad sensor. If you are not getting a voltage and the car has been running rich lately, the sensor may be carbon fouled. It is sometimes possible to clean a sensor in the car. Do this by unplugging the sensor harness, warming up the engine, and creating a lean condition at about 2000 rpm for 1 or 2 minutes. Create a big enough vacuum leak so that the engine begins to slow down. The extra heat will clean it off if possible. If not, the sensor is fried. In either case, fix the cause of the rich mixture and retest. If you don't, the new sensor will fail.

My car has two (or more) wires coming out of the sensor, which wire is the signal output? 

     Most modern cars used a heated oxygen sensor. In this case, there will be two three or four wires instead of one. What the heating element does is heat the oxygen sensor up quicker than simply relying on the heat of the exhaust manifold.

     If you have two, three, or four wires, use a 15 or higher volt scale on the meter to test each wire for 12 volts. You will need to determine which wire is the signal output wire. Once you have determined which wire is the output signal wire, test the sensor as shown above. 

     A heated oxygen sensor with two wires is usually wired like this (pic o2_sensor_diagram_2.jpg) One wire is 12 volts for the heating element. The other wire is the signal output wire to the fuel injection computer.  In this case, the oxygen sensor casing is the ground.

     With a 3 wire oxygen sensor, it is wired like this, (pic  o2_sensor_diagram_3.jpg) one wire is 12 volts for the heating element, one is ground for the heating element, and the last wire is the signal output to the fuel injection computer.

     Lastly, with a 4-wire oxygen sensor, the output signal works in a constant loop, essentially, the fuel injection computer sends a signal to the sensor, and the sensor then sends the signal back to the computer. This is achieved by having one wire carry the signal to the sensor, and then another wire carries the signal back to the fuel injection computer. You also have a wire carrying 12 volts to the heating element, and the last wire is the ground for the heating element. (pic  o2_sensor_diagram_4 jpg) With a 4-wire sensor, you will need to measure the voltage fluctuations between the two signal wires. To do this, start the car and let it warm up. Next, disconnect the oxygen sensor and measure the voltage between the wires. Now take off the oil filler cap to simulate a rich running condition. Watch the voltage, if it begins to fluctuate rapidly, then the sensor is good and you’re done. If it stays the same, it’s probably fried.

     Well, there you have it - it's really not too difficult at all.  If you would like to see more technical articles like this one, please continue to support Pelican Parts with all your parts needs.  If you like what you see here, then please visit our online BMW catalog and help support the collection and creating of new and informative technical articles like this one.  Your continued support directly affects the expansion and existence of this site and technical articles like this one.  As always, if you have any questions or comments about this helpful article, please drop us a line.