- Oxygen Sensors

Did you know that oxygen sensors, like spark plugs, should be replaced regularly to keep a vehicle running cleanly and efficiently?

If you did, count yourself among the educated few. If you didn't, don't feel bad; you're in very good company. Few motorists even know what an oxygen sensor is, or that their car has one, much less that it should be the subject of regular checks and periodic replacement. Even professional mechanics are not universally aware of the expected life of the various typres of oxygen sensors found on all of the vehicles they work on.

In fact, if you read this article and learn the information in it, you're likely to end up ranked in the 99th percentile range nationally, as far as oxygen sensors are concerned - that is, you'll know more about oxygen sensors than 99% of the U.S. population.

(By the way, in discussions of oxygen sensors, you will often see the word oxygen abbreviated by its chemical formula, O2, and hear it called "oh-two")

What It Does
An oxygen sensor's job is to monitor the amount of oxygen in a vehicle's exhaust gas as the gas leaves the combustion chamber, and to generate an electrical signal proportional to the amount of oxygen it finds. It then sends the signal to the vehicle's electronic control module (or ECM, the onboard engine management computer), which adjusts the fuel/air mixture if the sensor signal indicates the engine is running too rich or too lean.

To keep the engine running at peak efficiency, and to prevent unacceptable levels of exhaust emissions, the O2 sensor must be able to react instantly to changes in oxygen content, and to generate a signal that accurately reflects the current level.

How It Works
O2 sensors are typically mounted in the exhaust manifold, just after the individual cylinder exhaust ports are channeled into a single tube. The sensor screws into a threaded hole in the manifold, so that the sensor tip (also called the probe, or electrode), protrudes into the exhaust flow. The sensor body and wires remain outside the manifold.

The probe tip, which is shaped like a small rocket nose cone, is made of high-temperature ceramic that is coated with a very thin layer of platinum that forms an electrode. The tip is protected by a slotted metal tube that allows exhaust gas to flow across the outside surface of the probe. Provision in the sensor body or wiring assures that the ambient air fills the inside of the probe tip.

At high temperatures (>300 degrees C, or 572 degrees F) the difference between the number of ionized particles in the exhaust gas on one side of the sensor probe and the number of ionized particles in the ambient (reference) air on the other side of the sensor probe generates a potential difference (voltage) across the probe tip. The higher the number of ionized particles in the exhaust gas compared to the ambient air, the higher the voltage generated.

The amount of ionization in the exhaust gas goes up as the amount of unburned fuel goes up. That means that when the fuel/air mixture is too rich, the sensor generates a higher voltage (about 800 millivolts, or 0.8 volts), and when the mixture is too lean, the sensor generates a lower voltage (about 200 millivolts, or 0.2 volts).

To compensate, the vehicle's ECM is programmed to increase the fuel charge (make the mixture richer) when it receives a low voltage signal form the O2 sensor, and ti decrease the fuel charge (make the mixture leaner) when it receives a high voltage signal from the O2 sensor. By responding quickly to changes in exhaust chemistry , the O2 sensor can keep the engine running very near its peak efficiency under nearly all driving conditions. A healthy sensor will detect and react to a change in oxygen content within about 100 milliseconds (0.1 sec.).

The initial oxygen sensor design was simpler than today's versions. It consisted of the probe, the housing, and a single wire used to transmit the voltage signal to the ECM. While that design improved engine operation and helped to lower emissions compared to engines without an O2 sensor, it had some drawbacks.

First, the sensor only generates a voltage after it reaches around 300 degrees C (572 degrees F), so the initial-design sensors require several minutes after a cold start to warm up. During that time, the engine can run too rich or too lean, and exhaust emissions remain uncontrolled.

Furthermore, the tip temperature can fluctuate during normal operation from a low of about 400 degrees C (752 degrees F) with the engine at idle to as much as 900 degrees C (1652 degrees F) with the engine under full load. That 500 degrees C temperature swing puts significant thermal stress on the sensor probe tip.

To help solve these problems, later model sensors include a heating element that electrically heats the probe tip to help it warm up faster, and to level out the temperature swings it's subjected to under normal operation. These newer sensors can be spotted by the number of wires coming from the back of the housing (three or four, instead of just one).

Single-wire, unheated sensors were used on some vehicles from as early as 1976 (the date of the first application of oxygen sensors) to the early '90s. First generation heated sensors found their way into some vehicles in the mis-'80s, and are still being used in some applications.

Second generation heated sensors are just now being applied to new vehicles and will continue in use for some period of time. While the second generation sensors operate in basically the same way as the first generation heated sensors, they contain structural improvements, and offer a longer operating life (see the recommended change intervals in Table 1).



What Can Go Wrong
As noted earlier, a properly operating O2 sensor can respond to changes in exhaust gas composition (go from a low voltage, 200 mV signal to a high voltage, 800 mV signal) in about 100 milliseconds.

Problems arise, though, when the O2 sensor is no longer able to respond that fast, or to measure accurately the amount of oxygen in the exhaust. Under those conditions, the engine no longer operates efficiently and rich or lean conditions can occur for protracted periods of time.

The result is poor driveability, manifested by one or more of the following symptoms: decreased fuel economy, hesitation on acceleration, stalling, surging, rough idle, and increased tailpipe emissions (with a likely failure of emissions tests, where mandated).

In addition to driveability problems, a faulty O2 sensor can lead to premature failure on the catalytic converter.

The most common causes of early sensor failure are deposits on the probe tip that prevent the tip from accurately measuring the amount of oxygen in the exhaust gas. Silicone, condensed water, and some oil additives can contaminate the sensor.

In addition, sensors can be subjected to extreme temperatures, oil fouling, carbon deposits, and the corrosive effects of a myriad of harmful chemicals during a "normal" life. Eventually, even the best oxygen sensor, operating in the cleanest engine, will wear out.

The Solution
The simple solution to better engine performance and emissions compliance is the replacement of the oxygen sensor at intervals prescribed by the sensor or vehicle manufacturer. At present, aftermarket sales data indicate that just over a fourth of the O2 sensors that should be replaced are actually being replaced.

Sensor manufaturers also note that O2 sensors can be working poorly, without causing symptoms noticeable to the driver. An oscilloscope test will reveal response time, and a slow sensor, before it's bad enough to cause harsh symptoms.

Sensors screw in and out of the manifold easily, so most do-it-yourselfers can make the change, following instructions included with the sensor. Just remember that a special O2 sensor socket that's slotted to prevent damage to the sensor wiring will make the job a lot easier.

Also, the wire colors from the new sensor to the wiring harness must always be paired exactly, and the sensor threads should be coated with a conductive antiseize compound to make eventual removal safer, with less likelihood of stipping the threads in the exhaust manifold (possibly requiring manifold replacement).