 | Your Oxygen sensor or o2 sensor measures the amount of the oxygen in the exhaust gases. This information is used by the automotive engine computer system to control engine operation. There are a few types of oxygen sensors available, but here we will consider most commonly used - voltage-generating type. |
 Single-wire oxygen sensor | |
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The amount of oxygen the engine can pull in depends on factors such as the altitude and the temperature of the air and engine.
A gasoline engine burns gasoline in the presence of oxygen (see How Car Engines Work for complete details). It turns out that there is a particular ratio of air and gasoline that is "perfect," and that ratio is 14.7:1 (different fuels have different perfect ratios -- the ratio depends on the amount of hydrogen and carbon found in a given amount of fuel). If there is less air than this perfect ratio, then there will be fuel left over after combustion. This is called a rich mixture. Rich mixtures are bad because the unburned fuel creates pollution. If there is more air than this perfect ratio, then there is excess oxygen. This is called a lean mixture. A lean mixture tends to produce more nitrogen-oxide pollutants, and, in some cases, it can cause poor performance and even engine damage.
The oxygen sensor or o2 sensor is positioned in the exhaust pipe and can detect rich and lean mixtures. The mechanism in most sensors involves a chemical reaction that generates a voltage (see the patents below for details). The engine's computer looks at the voltage to determine if the mixture is rich or lean, and adjusts the amount of fuel entering the engine accordingly.
The reason why the engine needs the oxygen sensor or o2 sensor is because the amount of oxygen that the engine can pull in depends on all sorts of things, such as the altitude, the temperature of the air, the temperature of the engine, the barometric pressure, the load on theengine, etc.
When the oxygen sensor or o2 sensor fails, the computer can no longer sense the air/fuel ratio, so it ends up guessing. Your car performs poorly and uses more fuel than it needs to.
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In response to several requests for more information about Oxygen (O2) sensors, perhaps the following information will help. Comment:
These procedures are only for self powered conventional oxygen or o2 sensor sensors. Some very new cars are using a different style sensor that is powered. *Many* Oxygen sensors are replaced that are good to excellent. *Many* people don't know how to test them. They routinely last 50,000 or more miles, and if the engine is in good shape, can last the life of the car.What does the O2 sensor do?
It is the primary measurement device for the fuel control computer in your car to know if the engine is too rich or too lean. The O2 sensor is active anytime it is hot enough, but the computer only uses this information in the closed loop mode. Closed loop is the operating mode where all engine control sensors including the Oxygen sensor are used to get best fuel economy, lowest emissions, and good power.Should the O2 sensor be replaced when the sensor light comes on in your car?
Probably not, but you should test it to make sure it is alive and well. This assumes that the light you see is simply an emissions service reminder light and not a failure light. A reminder light is triggered by a mileage event (20-40,000 miles usually) or something like 2000 key start cycles. EGR dash lights usually fall into the reminder category. Consult your owners manual, auto repair manual, dealer, or repair shop for help on what your light means.How do I know if my O2 sensor may be bad?
If your car has lost several miles per gallon of fuel economy and the usual tune up steps do not improve it. This *is not* a pointer to O2 failure, it just brings up the possibility. Vacuum leaks and ignition problems are common fuel economy destroyers. As mentioned by others, the on board computer may also set one of several failure "codes". If the computer has issued a code pertaining to the O2 sensor, the sensor and it's wiring should be tested. Usually when the sensor is bad, the engine will show some loss of power, and will not seem to respond quickly.What will damage my O2 sensor?
Home or professional auto repairs that have used silicone gasket sealer that is not specifically labeled "Oxygen sensor safe", "Sensor safe", or something similar, if used in an area that is connected to the crankcase. This includes valve covers, oil pan, or nearly any other gasket or seal that controls engine oil. Leaded fuel will ruin the O2 sensor in a short time. If a car is running rich over a long period, the sensor may become plugged up or even destroyed. Just shorting out the sensor output wire will not usually hurt the sensor. This simply grounds the output voltage to zero. Once the wiring is repaired, the circuit operates normally. Undercoating, antifreeze or oil on the *outside* surface of the sensor can kill it. See how does an Oxygen sensor work.Will testing the O2 sensor hurt it?
Almost always, the answer is no. You must be careful to not *apply* voltage to the sensor, but measuring it's output voltage is not harmful. As noted by other posters, a cheap voltmeter will not be accurate, but will cause no damage. This is *not* true if you try to measure the resistance of the sensor. Resistance measurements send voltage into a circuit and check the amount returning.How does an O2 sensor work?
An Oxygen sensor is a chemical generator. It is constantly making a comparison between the Oxygen inside the exhaust manifold and air outside the engine. If this comparison shows little or no Oxygen in the exhaust manifold, a voltage is generated. The output of the sensor is usually between 0 and 1.1 volts. All spark combustion engines need the proper air fuel ratio to operate correctly. For gasoline this is 14.7 parts of air to one part of fuel. When the engine has more fuel than needed, all available Oxygen is consumed in the cylinder and gasses leaving through the exhaust contain almost no Oxygen. This sends out a voltage greater than 0.45 volts. If the engine is running lean, all fuel is burned, and the extra Oxygen leaves the cylinder and flows into the exhaust. In this case, the sensor voltage goes lower than 0.45 volts. Usually the output range seen seen is 0.2 to 0.7 volts. The sensor does not begin to generate it's full output until it reaches about 600 degrees F. Prior to this time the sensor is not conductive. It is as if the circuit between the sensor and computer is not complete. The mid point is about 0.45 volts. This is neither rich nor lean. A fully warm O2 sensor *will not spend any time at 0.45 volts*. In many cars, the computer sends out a bias voltage of 0.45 through the O2 sensor wire. If the sensor is not warm, or if the circuit is not complete, the computer picks up a steady 0.45 volts. Since the computer knows this is an "illegal" value, it judges the sensor to not be ready. It remains in open loop operation, and uses all sensors except the O2 to determine fuel delivery. Any time an engine is operated in open loop, it runs somewhat rich and makes more exhaust emissions. This translates into lost power, poor fuel economy and air pollution. The O2 sensor is constantly in a state of transition between high and low voltage. Manfucturers call this crossing of the 0.45 volt mark O2 cross counts. The higher the number of O2 cross counts, the better the sensor and other parts of the computer control system are working. It is important to remember that the O2 sensor is comparing the amount of Oxygen inside and outside the engine. If the outside of the sensor should become blocked, or coated with oil, sound insulation, undercoating or antifreeze, (among other things), this comparison is not possible.How can I test my O2 sensor?
They can be tested both in the car and out. If you have a high impedence volt meter, the procedure is fairly simple. It will help you to have some background on the way the sensor does it's job. Read how does an O2 sensor work first.Testing O2 sensors that are installed
The engine must first be fully warm. If you have a defective thermostat, this test may not be possible due to a minimum temperature required for closed loop operation. Attach the positive lead of a high impedence DC voltmeter to the Oxygen sensor output wire. This wire should remain attached to the computer. You will have to back probe the connection or use a jumper wire to get access. The negative lead should be attached to a good clean ground on the engine block or accessory bracket. Cheap voltmeters will not give accurate results because they load down the circuit and absorb the voltage that they are attempting to measure. A acceptable value is 1,000,000 ohms/volt or more on the DC voltage. Most (if not all) digital voltmeters meet this need. Few (if any) non-powered analog (needle style) voltmeters do. Check the specs for your meter to find out. Set your meter to look for 1 volt DC. Many late model cars use a heated O2 sensor. These have either two or three wires instead of one. Heated sensors will have 12 volts on one lead, ground on the other, and the sensor signal on the third. If you have two or three wires, use a 15 or higher volt scale on the meter until you know which is the sensor output wire. 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 your connections. Next, check your leads to make sure you won't wrap up any wires in the belts, etc. then start the engine. You should run the engine above 2000 rpm for two minutes to warm the O2 sensor and try to get into closed loop. Closed loop operation is indicated by the sensor showing several cross counts per second. It may help to rev the engine between idle and about 3000 rpm several times. The computer recognizes 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 (> 0.45) near 0.45 or steady low (<>Testing O2 sensors on the workbench. Use a high impedence DC voltmeter as above. Clamp the sensor in a vice, or use a plier or vice-grip to hold it. Clamp your negative voltmeter lead to the case, and the positive to the output wire. Use a propane torch set to high and the inner blue flame tip to heat the fluted or perforated area of the sensor. You should see a DC voltage of at least 0.6 within 20 seconds. If not, most likely cause is open circuit internally or lead fouling. If OK so far, remove from flame. You should see a drop to under 0.1 volt within 4 seconds. If not likely silicone fouled. If still OK, heat for two full minutes and watch for drops in voltage. Sometimes, the internal connections will open up under heat. This is the same a loose wire and is a failure. If the sensor is OK at this point, and will switch from high to low quickly as you move the flame, the sensor is good. Bear in mind that good or bad is relative, with port fuel injection needing faster information than carbureted systems. ANY O2 sensor that will generate 0.9 volts or more when heated, show 0.1 volts or less within one second of flame removal, AND pass the two minute heat test is good regardless of age. When replacing a sensor, don't miss the opportunity to use the test above on the replacement. This will calibrate your evaluation skills and save you money in the future. There is almost always *no* benefit in replacing an oxygen sensor that will pass the test in the first line of this paragraph.But if you must replace the oxygen or o2 sensor, here is some great tips to help you out.
Twelve years and more than 100,000 miles have passed under your trusty commuter and the Check Engine light has never, ever, winked at you … until yesterday, when it coincidentally anticipated your state inspection appointment at the end of the month. Rats! The car will never pass the emissions test with that light on. Now what?
Here’s the perfect opportunity to break out that new, easy-to-use, consumer-grade OBD II (On-Board Diagnostics II) generic code reader. That’s what we did on our sacrificial lamb, a Nissan Altima. After plugging in the universal connector under the dash, we retrieved a code P0136 “O2 Sensor Circuit Malfunction (Sensor 2).” This let us zero in on the likely problem right away. Remember, a trouble code stored in the engine computer doesn’t necessarily tell you what’s wrong. It’s just a good starting point.
So, it’s time to actually check out the sensor. Sensor 2 is the downstream sensor, in the catalytic converter, smack in the middle of the underside of the car. Start by getting the car up on some safety stands, then roll underneath it with a creeper.
Lazy O2 Sensor Equals Low MPG
OBD I engine management, dating back to 1980, used just one upstream O2 sensor, mounted in the exhaust manifold, as close as possible to the cylinder head’s heat. That was because an oxygen sensor can’t produce and send the rapidly toggling voltage signal the engine computer is expecting until the sensor is really hot (above 600 F). That’s why today all O2 sensors are electrically heated — so they will start working sooner.It’s always the upstream sensor (“Bank 1, Sensor 1”) that the powertrain management system pays attention to in order to fine-tune the proper 14.7:1 air/fuel ratio (aka lambda). Lambda is the Greek character used to designate that perfect stoichiometric ratio. And although the modern oxygen sensor has a 100,000-mile life expectancy, when it gets old and lazy you’ll begin to notice a drop in fuel economy. More extreme cases of malfunction will lead to driveability issues and, eventually, to an illuminated Check Engine light when the frequency of the sensor’s signal slows to a crawl.
Rules of Thumb
It is possible to do some simple checks on O2 sensors with a high-impedance digital voltohmmeter. You don’t necessarily need a professional technician’s scan tool. But to perform specific test procedures, it definitely helps to have the service manual for your vehicle. Some manuals, for example, give simple static resistance measurements across the sensor’s terminals. These alone may not be conclusive.Do a dynamic test. Determine the frequency at which a good sensor is supposed to toggle back and forth from 0 to 1 volt while the engine is being revved. A general rule of thumb says the sensor should toggle two to three times per second at 2500 rpm. There are variations: Nissan says the rear heated oxygen sensor we replaced should read above 0.6 volt at least once while racing the engine up to 4000 rpm (under no load). Your service manual will have the specifics for your vehicle.
Or, you can just replace your O2 sensor(s) at regular intervals, to try and prevent that minor drop in mpg sometimes caused by a “lazy” high-mileage sensor.
Our problem was obvious once we got under the car. The sensor’s harness had snagged on some road debris or, more likely, high-centered on a pile of ice here in New York last winter. With the harness dangling in the breeze, not connected to anything, it’s no wonder the Check engine light was on.
But instead of just fixing the harness, we chose to go with a new $100 sensor. Its fresh electrical connector brought peace of mind. Besides, the old unit had been in service for 104,000 miles, so it was due for replacement. Despite the rust caked around the sensor’s mount, a good yank with an open-end wrench was all it took to break it loose.
We got lucky, because thiso2 sensor was easy to get at. More often than not, you’ll need to acquire one of the specially slotted oxygen sensor sockets, available wherever mechanic’s tools are sold. The slot in the side of these sockets enables easy removal and installation, even in cramped engine compartments, and protects the sensor’s wire leads.
Finally, after the sensor fix and a certain amount of driving and key cycling, we were waiting for the Altima’s Check Engine light to go out by itself. It didn’t. So we had to diagnose a little further. At last, we found two weak engine grounds. Their terminals had oxidized slightly on the aluminum intake manifold and were causing enough of a voltage drop in the ground side of the sensor circuit to keep the engine computer on guard. This caused two additional codes to store in the system along with the O2 sensor code. Ultimately, loosening and retightening the two ground strap bolts allowed us to clear all the codes from memory. The Check Engine light blinked off and the car passed its emissions test with flying colors.
1] Sometimes you get lucky: The old sensor unscrewed easily.
2] A sparing dollop of antiseize will make the next removal easier. New sensors already have some on the threads.
3] Thread the new harness back into the body of the car. Make sure the grommet seals out road dirt and water. Use cable ties to keep the harness tucked
out of the way.
If your oxygen sensor or o2 sensor fails, replacing them can be costly unless you have the know how to do it your self.
out of the way.
If your oxygen sensor or o2 sensor fails, replacing them can be costly unless you have the know how to do it your self.
