From Bosch "REPORTER Written by Technicians for Technicians," October 1998:


Automotive exhaust emissions are everyone's concern because we all breathe the same air. Fifty percent of Americans live in areas which exceed national clean air standards. Reducing tailpipe emissions, therefore, is a top priority in the effort to fight air pollution.

In 1976, Bosch introduced what would eventually become one of the most important technologies for reducing exhaust emissions: the oxygen sensor. By 1996, Bosch had produced its 100 millionth oxygen sensor. Today, Bosch oxygen sensors are original equipment on a wide variety of European, Asian and domestic vehicles and are the No. 1 best selling brand in the aftermarket.

Oxygen sensors have been standard equipment on passenger car and light truck engines since 1980-81/ Most such vehicles have one or two oxygen sensors (two are typically used on selected V6 and V8 engines starting in the late 80s). Since the introduction of Onboard Diagnostics II (OBD II) in 1995-96, the number of oxygen sensors per vehicle has doubled (the extra sensors are used downstream of the catalytic converter to monitor its operating efficiency). Yet, as important as oxygen sensors are today, few motorists are even aware of their presence - let alone the key role oxygen sensors play in engine performance and reducing pollution. One survey found that 99.7% of all consumers did not know their vehicle had an oxygen sensor!


Originally called a "Lambda sensor" when it was first used in fuel-injection European applications, the oxygen sensor monitors the level of oxygen (O2) in the exhaust so an onboard computer can regulate the air/fuel mixture to reduce emissions. The sensor is mounted in the exhaust manifold and generates a voltage signal proportional to the amount of oxygen in the exhaust. The sensing element on 99% of all oxygen sensors in use is a zirconium ceramic bulb coated on both sides with a thin layer of platinum. The outside of the bulb is exposed to the hot exhaust gases, while the inside of the bulb is vented internally through the sensor body or wiring to the outside atmosphere.

When the air/fuel mixture is rich and there is little O2 in the exhaust, the difference in oxygen levels across the sensing element generates a voltage through the sensor's platinum electrodes: typically 0.8 to 0.9 volts. When the air/fuel mixture is lean and there is more oxygen in the exhaust, the sensor's voltage drops to 0.1 to 0.3 volts. When the air/fuel mixture is perfectly balanced and combustion is cleanest, the sensor's output voltage is around 0.45 volts.

The oxygen sensor's voltage signal is monitored by the onboard engine management computer to regulate the fuel mixture. When the computer sees a rich signal (high voltage) from the O2 sensor, it commands the fuel mixture to go lean. When the computer receives a lean signal (low voltage) from the O2 sensor, it commands the fuel mixture to go rich. Cycling back and forth from rich to lean averages out the overall air/fuel mixture to minimize emissions and to help the catalytic converter operate at peak efficiency (which is necessary to reduce hydrocarbon (HC), carbon monoxide (CO) and oxides of nitrogen (NOX) levels even further).

The speed with which the oxygen sensor reacts to oxygen changes in the exhaust is very important for accurate fuel control, peak fuel economy, and low emissions. The air/fuel mixture in an older carbureted engine doesn't change as quickly as that in a throttle body fuel-injected application, so response time is less critical. But, in newer engines with multipoint fuel injection, the air/fuel mixture can change extremely fast, requiring a very quick response from the o2 sensor.


Nothing lasts forever, and oxygen sensors are no exception. As an oxygen sensor, contaminants from normal combustion and oil ash accumulate on the sensing element. This reduces the sensor 's ability to respond quickly to changes in the air/fuel mixture. The sensor slows down and becomes "sluggish." At the same time, the sensor's output voltage may not be as high as it once was, giving the false impression that the air/fuel mixture is leaner than it actually is. The result can be a richer-than-normal air/fuel mixture under various operating conditions that causes fuel consumption and emissions to rise.

The problem may not be notice right away because the change in performance occurs gradually. But, over time, the situation will get worse, ultimately requiring the sensor to be replaced to restore peak engine performance.


The normal aging process will eventually cause the oxygen sensor to fail. However, the sensor, may also fail prematurely if it becomes contaminated with lead from leaded gasoline, phosphorous from excessive oil consumption, or silicone from internal coolant leaks or using silicone sprays or gasket sealers on the engine. Environmental factors such as road splash, salt, oil, and dirt can also cause a sensor to fail, as can mechanical stress or mishandling.

A dead sensor will prevent the onboard computer from making the necessary air/fuel corrections, causing the air/fuel mixture to run rich in the "open loop" mode f operation, resulting in much higher fuel consumption and emissions.

An additional consequence of an O2 sensor failure may be damage to the catalytic converter. A rich operating condition causes the converter to run hotter than normal. If the converter gets hot enough, the catalyst substrate inside may actually melt forming a partial or complete blockage. The result can be a drastic drop in highway performance or stalling because of a buildup of backpressure in the exhaust system.


Although some vehicles have an oxygen sensor "reminder" light to alert the driver when it is time to check the oxygen sensor, most do not. So, unless there's a noticeable driveability problem or a "Check Engine" light on, most motorists have no way of knowing if their oxygen sensor is functioning properly or not.

The growth of emissions testing nationwide is changing that, along with the introduction of new "enhanced" emissions testing programs that simulate real world driving conditions while emissions are being measured. The latest is proving to be very effective at catching emissions problems that formerly escaped detection.

According to a study conducted by Sierra Research, Inc., in 1996, oxygen sensor failure is the "single greatest source of excessive emissions for fuel-injected vehicles" and the second most significant cause of high emissions in carbureted engines.

The U.S. Environmental Protection Agency (EPA) and the California Air Resource Board (CARB) have found that oxygen sensor replacement was required on 42-58% of all vehicles that were subjected to an emissions check and found to be emitting high level s of hydrocarbons (HC) or Carbon monoxide (CO). Checking the operation of the oxygen sensor and the feedback system, therefore, should always be a priority anytime a vehicle fails an emissions test due to high HC or CO.

Sensor performance can be checked by reading the sensor's output voltage to make sure it corresponds with the air/fuel mixture (low when lean, high when rich). The voltage signal can also be displayed as a waveform on an oscilloscope to make sure the signal is changing back and forth from rich to lean and is responding quickly enough to changes in the air/fuel ratio


To minimize the consequences of normal aging, Bosch recommends oxygen sensor replacement for preventative maintenance at the following intervals:

Unheated O2 sensors on 1976 to early 1990s application: every 30,000 to 50,000 miles.
Heated (1st generation) O2 sensors on mid-1980's to mid-1990's applications: every 60,000 miles.
Heated (2nd generation) O2 sensors on mid-1990s and up applications: every 100,000 miles.
Keeping the sensor fresh may improve fuel economy as much as 10-15% (which can save $100 each year in fuel costs on the average). Keeping the sensor in good operating condition will also minimize exhaust emissions, reduce the risk of costly damage to the catalytic converter, and ensure peak engine performance (no surging or hesitating).

For these reasons, the O2 sensor should be considered a "tune-up" replacement item just like spark plugs, especially on older vehicles (those built before the mid-1990's).

Bosch oxygen sensors are an exact replacement for the original. The construction, number of wires and connectors are the same as the original, which eliminates the risks associated with splicing and crimping wires (required for many "universal replacement O2 sensors). Some three - or four-wire universal O2 sensors also do not have the same heater circuit watt ratings as the original sensor, which may cause driveability and emissions problems. There is also a potential for damaging the computer and/or O2 sensor if a multiwire universal sensor is connected incorrectly. The lack of a standardization of wire colors increases the risk of an incorrect installation.