Oxygen Sensors - Theory & Operation

This report was prepared by: Bill Brown, Travis Lafon, Ben Perry & Chris Schonbrod

The O2 sensor gets its name from the chemical name for oxygen, O2. Oxygen atoms always travel in pairs never alone. The oxygen sensor does only one job, it creates an electronic signal exactly proportionate to the amount of oxygen contained in the exhaust gases of an engine. The PCM (Powertrain Control Module) monitors this signal and uses it to compute air to fuel ratios for the engine.

On one end, oxygen sensors have a zirconium ceramic bulb. This end is threaded into the exhaust manifold/piping of the engine, before it reaches the catalytic converter. Inside the bulb are two strips of platinum that serve as electrodes or contacts. The outside of the bulb is exposed to the exhaust gases while the inside of the bulb is vented internally through the sensor to the outside air. Older style oxygen sensors have a small hole in the body so air can enter the sensor, but newer style sensors breathe through their wire connectors and have no vent hole. Grease should never be applied to the wires where they enter the sensor, because it would prevent proper air circulation. So will dirt and other contaminates. Because of the difference in oxygen and temperature levels between the inside and outside of the bulb, a small voltage is created and can be measured by the PCM.

Oxygen sensors must reach an operating temperature of about 600 degrees before they will begin to create any voltage. For this reason, some O2 sensors have three or fours wires instead of the traditional one or two. The extra wires are for a small heating element inside the O2 sensor to help it reach operating temperature quicker. (And sometimes to help keep it there during idle when exhaust temperatures are lower than normal.) Oxygen sensors create more voltage when there is less oxygen in the exhaust. Example: When the engine is running rich, burning a lot of fuel and in the process burning up all of the oxygen. The maximum output for almost any O2 sensor is about one volt. Oxygen sensors create less voltage when there is more oxygen in the exhaust. Example: Engine is running lean or misfiring, letting larger amounts of air and unburnt oxygen to pass through the system. Leaking exhaust manifold gaskets, vacuum lines and other items can also let excess air get into the exhaust and add more oxygen creating a lower voltage signal. When the mixture is lean the sensor voltage will drop down to about .1 volts.

When the computer sees a rich (high) signal it reduces the amount of fuel. When it sees a lean (low) signal it increases the amount of fuel. The rate at which the PCM changes back and forth between rich and lean varies depending on the engine. This process is known as cycling. Engines with feedback carburetors usually cycle once per second at 2500 rpm. Engines with throttle body fuel injection cycle two to three times per second at 2500 rpm and engines with multi-port fuel injection cycle five to seven times per second at 2500 rpm.

Oxygen sensors play a big role in fuel control because they bring the fuel feedback control loop into what is known as closed loop. The PCM monitors the O2 sensors output and adjusts the fuel accordingly. In open loop, the PCM has a set value for fuel control. Open loop is used when the vehicle has not yet reached operating temperature. (The O2 sensor is still cold and isn't putting out a signal yet.) Or when there is a problem in the O2 sensor circuit or the O2 sensor itself. The primary reason for the engine to cycle back and forth between rich and lean is to allow the catalytic converter to operate at peak efficiency, thereby drastically lowering vehicle emissions. Several other factors (or sensors) also contribute to the PCM switching from open loop to closed loop. Things like coolant temperature, air temperature, vehicle load, atmospheric pressure and throttle position. A bad coolant temperature sensor, for instance, could keep the feedback control loop in open loop at all times by making the PCM think the engine is always cold.

Oxygen sensors tend to become sluggish with age and do require periodic replacement. Contaminants begin to accumulate on the sensor tip and reduce its ability to create voltage. Contaminants could be lead, silicone, sulfur, oil ash and some fuel additives. These can come from burning leaded fuel, oil or antifreeze. Typical replacement is recommended every 35,000-50,000 miles. Although some newer vehicle's (1996 and up) O2 sensors have up to a 100,000 mile service interval. A recent EPA study found that 70 percent of vehicles that failed an I/M240 emissions test needed a new O2 sensor.

Many newer vehicles also have oxygen sensors after the catalytic converter in addition to the one(s) in front. The purpose of these is to monitor the efficiency of the converter by measuring the amount of oxygen in the exhaust downstream from it. (There should be very little to none.) Some V6 and V8 cars with dual exhaust systems can have as many as four O2 sensors.

The best way to test the performance of an oxygen sensor is to capture a waveform pattern from it on a digital storage oscilloscope (DSO). This way you can analyze the trace and make sure the sensor is switching from near minimum voltage (.1 volt) to near maximum voltage (1 volt). Usually when a sensor goes bad it will continually put out very little or no voltage. Some PCM's will illuminate the check engine light if there is no voltage signal coming from the O2 sensor. In some instances a scan tool is required to obtain the diagnostic codes for this condition. Unfortunately most O2 sensors will continue to work good enough to not trigger a fault code, but poorly enough to leave the fuel feedback control loop in open loop and create very high emissions and poor fuel economy (This is why replacement should be maintained at the proper intervals.). When using a DSO, an appropriate time scale for your waveform would be 1 second or 500 milliseconds per division and an appropriate voltage scale would be .2 volts per division. Finding and back probing the wires for the O2 sensor is also fairly straight forward. It is usually a good idea to get an engine wiring diagram before proceeding so that you can be positive which wire to probe. Since newer O2 sensors can have up to four wires (two for the heating element) it will definitely save you some headaches. Also, never pierce the wire, always back probe at the nearest connector. Some O2 sensor wires are shielded and piercing the wire will ground the sensor and ruin the signal.

Below are some oxygen sensor waveform patterns and some explanations as to what is going on. If you want more information, one of the best online resources I have found is the Babcox Publications search engine at www.babcox.com/uhsedsearch.htm. This search engine will search twelve different automotive related magazines for whatever you enter in. For more information on O2 sensors simply type in oxygen sensors.

This waveform was captured from a 1999 Kia Sephia running at idle. Notice that the sensor is cycling from about .2 volts to about 1 volt as the engine is going from lean to rich. Each cycle is taking about 2 seconds.

This waveform was also captured from the Kia at a slightly higher RPM. Notice the increase in cycling. The cycles are quicker now, taking only about one second each. This is normal and is due to the increased RPM's.

This is the same Kia with one fuel injector unplugged. Notice how the waveform almost flat lines at about .2 volts. This is because of the large amount of oxygen that has been allowed to enter the exhaust from the cylinder that isn't firing. The PCM will interpret this signal as a lean signal and will allow more fuel to be delivered to the engine resulting in decreased fuel economy and increased emissions.

This waveform was captured from a 1991 Subaru Legacy. The throttle was rapidly pumped to force the engine rich and then closed to force it lean. The voltage goes high in the beginning (up to 1 volt) because of the increased fuel resulting in little unburnt oxygen. The voltage goes low (down to almost 0) after the throttle is closed because large amounts of air are still being pushed out of the engine with no fuel to burn up the remaining oxygen after abruptly closing the throttle. This is a simple way to force the engine artificially rich and lean.

This waveform was captured from a 1990 Lincoln Continental with two oxygen sensors. One in each exhaust manifold of the V6 engine. The top trace was from O2 sensor #2 and the bottom trace was from O2 sensor #1 with an unplugged fuel injector on that side of the engine. Notice how the PCM tries to correct the lean signal in #1, created by the unplugged fuel injector, by running rich. The trace goes up and down because even though the PCM is increasing the amount of fuel creating a higher signal, the misfiring cylinder still allows extra air into the exhaust creating the low voltage drops. The trace for #2 remains somewhat high because there are no misfiring cylinders on that side to allow more oxygen into the exhaust. It stays even at about .5 and is not cycling because it is also running a little rich due to the obvious misfire affecting the whole engine on the other side. There is nothing wrong with the sensor itself, only the engine's fuel control.