Determining When An Oxygen Sensor Needs Replaced
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Determining When An Oxygen Sensor Needs Replaced

When diagnosing any oxygen sensor, remember that outside air entering the exhaust system from an exhaust leak will obviously reduce an oxygen sensor’s indicated voltage output. It’s also important to know that oxygen sensors can become rich or lean biased due to problems like sensor contamination and faulty sensor grounds. Remember that, regardless of how well it tests, a biased sensor will not produce a stoichiometric air/fuel ratio. So, if the sensor is questionable, it should be replaced.


Oxygen sensors have been a part of the automotive maintenance scene since 1976, when feedback fuel controls were popularly introduced. By 1980, nearly every car and light truck was equipped with an oxygen sensor that allowed their computer-controlled fuel systems to operate in a “closed-loop,” “feedback” or “fuel control” mode.

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Early single-wire zirconia oxygen sensors are usually replaced at 30,000-50,000-mile intervals or should be tested when a “maintenance” warning light is illuminated. Back in the day, early oxygen sensors often failed due to tetra-ethyl lead and silicon (dirt) contamination. Early oxygen sensors also became contaminated with phosphorous contained in engine oil, glycol contained in engine coolant and vapors emanating from silicone gasket sealants. See Photo 1.

Modern zirconia and air/fuel ratio (AFR) ­oxygen sensors last much longer because most of the above contaminants have been removed from gasoline, engine oil and gasket sealants. Since vehicles are being driven much longer, zirconia and AFR sensors most often malfunction because their internal heater circuits fail or because their zirconia-based sensing elements eventually lose their sensitivity to rapid changes in the engine’s air/fuel ratio.

Oxygen sensor diagnosis requires an intimate knowledge of oxygen sensor terminology. The earliest version of the oxygen sensor was originally called a “lambda sensor,” because it could detect when an air/fuel mixture varies from an ideal 14.7:1 weight ratio. Ideally, 14.7 parts of air mixed with 1.0 parts of fuel will completely ­oxidize, leaving only water and carbon dioxide. The term “stoichiometric” describes the state in which chemically perfect combustion is achieved.


In contrast to stoichiometric, Lambda indicates when the actual air/fuel mixture varies from the chemically perfect 14.7:1 a/f ratio. Lambda 1 indicates a perfect 14.7:1 a/f ratio, while a Lambda number of less than 1 indicates insufficient air supply. A Lambda number that’s greater than 1 indicates insufficient fuel in the cylinder. See Photo 2.

Photo 2: The coat of soot on this unheated oxygen sensor reduces its sensitivity to changes in the air/fuel mixture ratio.

During the early 1980s, many engines didn’t achieve fuel control (open-loop) until the coolant warmed to 160° F and the oxygen sensor warmed to 600° F. Since most exhaust pollution occurs ­during and shortly after a cold engine startup, oxygen sensors were later equipped with electric heaters to quickly bring them up to operating temperature. In modern vehicles, the PCM enters fuel control (closed-loop) as soon as the oxygen sensor begins sending a readable voltage signal to the PCM. Although operating strategies vary according to application, the PCM generally activates the oxygen sensor’s heater circuit during cold startups. Heated oxygen sensors generally last between 60,000-100,000 miles.


The sensing element of most oxygen sensors is composed of a zirconium dioxide thimble coated on both sides with a thin layer of platinum. As mentioned above, zirconia sensors must reach 600° F ­operating temperature before they begin generating a voltage signal. In addition, the inside of the zirconia thimble must be exposed to oxygen, which reaches the thimble through a vented housing or through the sensor lead wire. Although very little oxygen is required, a coating of engine oil or grease can reduce the availability of oxygen to the inner thimble enough to affect the sensor’s accuracy.

When the actual a/f mixture is rich, the sensor generates a 0.8 to 0.9-volt signal to the PCM.  When the actual a/f signal is lean, the sensor generates a much lower voltage signal to the PCM. At stoichiometric or Lambda 1, the sensor generates about 0.450 volts. When the oxygen sensor indicates “rich,” the PCM reduces the fuel injector pulse width. When the O2 sensor indicates “lean,” the PCM increases the injector pulse width. See Photo 3.

Photo 3: The shield on this new Toyota oxygen sensor is clean. If fuel control is correct, the oxygen sensor shield should accumulate only a light coating of combustion byproducts.

But, in reality, the combustion process is seldom perfect because the fuel closest to the combustion chamber surface or between the piston and cylinder wall often doesn’t burn. Consequently, a small amount of partially burned fuel in the form of carbon monoxide and unburned fuel in the form of hydrocarbons remains to form pollutants in the exhaust gas stream.


To more accurately monitor fuel control, the PCM in modern systems switches the air/fuel ratio from about 0.2 volts to 0.8 volts, which is very close to stoichiometric. This switching process can easily be observed by using the voltage-graphing feature found on most scan tools. In contrast to using a labscope, the graphing sample rate might be too low on a scan tool to provide absolutely accurate information. Nevertheless, the scan tool graph will indicate the voltage switching range and relative activity of the zirconia sensor.

When diagnosing any oxygen sensor, remember that outside air entering the exhaust system from an exhaust leak will obviously reduce an oxygen sensor’s indicated voltage output. It’s also important to know that oxygen sensors can become rich or lean biased due to problems like sensor contamination and faulty sensor grounds. Remember that, regardless of how well it tests, a biased sensor will not produce a stoichiometric air/fuel ratio. So, if the sensor is questionable, it should be replaced.

In many post-1996 OBD II systems, the zirconia sensor has changed from a thimble-shaped configuration to what is known as a “planar” or flat configuration. The planar design allows a much shorter warm-up time, is more reliable and is more accurate over its operating life. On most current platforms, zirconia sensors are used downstream from the catalytic converter because the voltage reporting requirements are within the range of a zirconia sensor. See Photo 4.

Photo 4: The locating tabs make this Toyota oxygen sensor application-specific. The four terminals indicate that this is a zirconia sensor.

Just for the historical record, some manufacturers like Toyota used titania-based oxygen sensors to indicate rich or lean air/fuel mixtures. Unlike a zirconia sensor that produces voltage, the titania-based sensor is generally supplied with 5.0 reference volts. As a titania-based ­sensor heats up, it responds to variations from stoichiometric by changing resistance. Although the ­applications for titania-based sensors are relatively few, a technician must be able to recognize this configuration when a diagnosis is required.


AFR sensors are used in engines operating at ­extreme air/fuel ratios from 12:1 to 20:1 or higher. While AFR sensors are also known as “linear,” “broad-band,” “wide-band” and “lean” air/fuel ratio sensors, each of these design variations are generally application-specific and can generate a slightly different data stream.

Although an AFR sensor is basically two zirconia sensors or “cells” mated together in planar form, the AFR sensor uses an entirely different operating strategy than a conventional zirconia sensor. To remain within the scope of this article, suffice it to say that one cell is used to measure oxygen content in the exhaust stream and the other cell, known as a pumping cell, is supplied with a very small electric current capable of moving oxygen ions in a positive or negative direction. In so doing, this electric current achieves a stoichiometric ratio between both cells. The PCM therefore controls air/fuel ratio by measuring the amount of electric current flowing to and from the AFR sensor.


The differences between a conventional zirconia and AFR sensor are, first, that the AFR sensor will have five or more wires in its connector. Second, the AFR sensor must operate at 1,200° F, so it’s generally dependent upon its heater circuits to maintain operating temperature. And, in contrast to zirconia sensor diagnostics, AFR sensor diagnostics are, for practical purposes, scan-tool based.

If you’re using a factory or “enhanced” scan tool, you’re likely to see AFR data displayed in an entirely different format than on an aftermarket tool. Many aftermarket scan tools were mandated to display AFR data in a conventional 0 to 1-volt switching pattern format.

While this format is erroneous in one sense, it also becomes irrelevant in another because modern OBD II PCMs have a much greater and far more sophisticated on-board diagnostic capability than just a few years ago. So, in most cases, it’s much better to let the modern OBD II PCM run the diagnostic monitors on the AFR sensor and store the related trouble codes when the AFR sensor begins to degrade.


Because AFR sensors can detect a wide range of air/fuel ratios in the feed gases exiting the engine, they are generally used upstream of the catalytic converter. Again, remember that leaking exhaust manifolds or EGR systems will create a false AFR signal to the PCM.

Scan tools identify oxygen sensors according to cylinder bank and position. Number-one cylinder bank is the bank closest to the harmonic balancer on a V-block engine. In relation to the catalytic converter, the B1S1 oxygen sensor is the first or “upstream” oxygen sensor on the bank one side. B1S2 is the second sensor located downstream from the catalytic converter. Some systems use two upstream bank sensors per cylinder bank and are numbered accordingly.

Although it’s obvious that a new oxygen sensor is required when the PCM detects a failure, there are other occasions when an oxygen sensor replacement might be recommended. As mentioned at the outset, many older import vehicles are equipped with sensors that should be replaced at regular intervals or inspected when the vehicle’s orange “maintenance required” light illuminates.


Because pre-1996 OBD I vehicles lack the on-board diagnostic capability to detect a failing zirconia oxygen sensor, it’s always best to test sensor voltage range and sensitivity with a labscope or digital multimeter. When removed, the sensor shield should exhibit a nearly clean-metal ­appearance. If the sensor is crusted with oil ­contamination, it should be replaced and the engine tested for excessive fluid consumption.

Similarly, if an oxygen sensor has been exposed to coolant from a leaking cylinder head gasket, it should be replaced to ensure the PCM’s ability to establish correct fuel control.

Last, if the vehicle is failing an emissions test, remember that sensors can produce a biased voltage due to internal or external contamination, or a faulty ground connection. In any case, a questionable sensor should be replaced to ensure accurate fuel control. 


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