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Oxygen Sensors
Questions Answers
What does an oxygen sensor do? The oxygen sensor sends a signal to the engine computer based on the amount of oxygen in the exhaust gas. This signal is used by the engine ECU to fine-tune the mixture to the optimum level for maximum catalyst efficiency and longevity. A worn-out oxygen sensor can cause excessive gasoline consumption, elevated exhaust emissions, accelerated catalytic converter damage failures and cause engine performance problems such as surging and hesitating.
Except for the connector assembly, aren't all oxygen sensors basically the same? NO. There are five fundamentally different types of oxygen sensors: unheated thimble, heated thimble, planar, wide-band and titania. Within each sensor type, sensors vary in the design of the ceramic element, heater element and protection tube design, all of which affect sensor operation.

Unheated Thimble: This was the first oxygen sensor design and Bosch has been producing them since 1976. These sensors either have one wire or two (for purposes of grounding), since they do not require power for a heater element. They use a zirconia ceramic element to generate a voltage in a rich (insufficient oxygen) gas environment. They can take several minutes to reach operating temperatures of 625 degrees Fahrenheit after a cold start, however this varies with application as the location of the sensor determines the rate at which it heats.
Click here for a diagram of a thimble type oxygen sensor.

Heated Thimble: These sensors were introduced by Bosch in 1982 and function in the same manner as the unheated thimble, however inside the thimble is a special heating element that brings the sensor up to operating temperature in about 30-60 seconds. The additional wires to power the heater- typically a total of 3-4 wires (dependent on grounding scenario) can identify these sensors.
Click here for a diagram of a thimble type oxygen sensor.

Planar: This sensor was introduced by Bosch in 1997 and also uses the zirconia ceramic sensing technology, but placed in a more compact thin flat shape (hence the name) instead of a thimble or conical shape. This allows the sensor to be smaller and lighter. They also require less electrical power as the heater is integrated into the smaller sensing element. By model year 2004, planar oxygen sensors are expected to account for over 30% of all new oxygen sensor applications and by 2007 for over 50% of new oxygen sensors. These sensors also have only four wires.
Click here for a diagram of a planar oxygen sensor.
Wideband: This is the newest design of oxygen sensor and it offers the increased accuracy needed to meet the latest emissions requirements. Unlike all the other types of oxygen sensors, the wide-band sensor can actually measure the air/fuel ratio from 11:1 (excess fuel condition) all the way to straight air (no fuel)! (The ideal air/fuel ratio is 14.7/1.) This improved measurement allows the engine control system to measure the actual air/fuel ratio and eliminates the switching between lean and rich associated with a traditional type of oxygen sensor. These sensors use a planar zirconia ceramic element, so that they heat up much faster than other types of sensors- reducing cold start emissions. These sensors can be identified with having five or more wires. In addition, these sensors are used with the newly developed gasoline direct injection engines. Direct injection engines can use stratified charges, which produce a very lean mixture in the combustion chamber, and these sensors must be used because of their ability to measure from very lean to very rich accurately.
Click here for a diagram of a wideband oxygen sensor. Titania: These sensors use a different type of oxygen sensing technology and instead of generating a voltage signal that changes with the air/fuel ratio, the sensor’s resistance changes. These sensors are used on less than 0.5% of all oxygen sensor equipped vehicles.
How do I know if my oxygen sensor may be damaged?

Unfortunately, the symptoms of a slow or even a dead sensor are not always obvious to the vehicle owner unless the vehicle fails an emissions test, a decline in fuel economy is noticed, or if driveability problems occur. Over time, exhaust contaminants accumulate on the sensing element reducing the sensor's ability to respond quickly to changes in air/fuel mixture and slowly the sensor becomes inoperable. Some symptoms of failed oxygen sensors are:

  • Failed emissions test (high CO and/or HC typically)
  • Damaged catalytic converter (caused by an over rich fuel mixture)
  • Poor fuel mileage (caused by an over rich fuel mixture)
  • Engine runs rough
  • Sluggish performance

If the “check engine” light comes on while driving, check your oxygen sensor, and if necessary replace it with a new premium Bosch Oxygen Sensor.

What will damage my oxygen sensor? An oxygen sensor can fail prematurely if it becomes contaminated with phosphorus from excessive oil consumption, silicone from internal coolant leaks, using silicone sealant in the engine, and some over-the-counter fuel additives. Even a small amount of poorly refined gasoline can kill an oxygen sensor. Environmental factors such as road splash, salt, oil, and dirt can also cause a sensor to fail, as can thermal shock, mechanical stress, or mishandling. However as required by vehicle manufacturers, Bosch sensors are designed and tested with these extremes in mind.
Click here to review damaged oxygen sensors.
How can I test my oxygen sensor? Unfortunately, the symptoms of a slow or even a dead sensor are not always obvious unless the vehicle fails an emissions test, a decline in fuel economy is noticed, or if driveability problems occur. Furthermore, while a dead sensor can be detected with a relatively inexpensive digital volt-ohmmeter, a slow sensor can only be diagnosed by a more expensive oscilloscope or scope meter.
Where are oxygen sensors located and do they have different purposes? Oxygen sensors have been standard equipment on almost all passenger cars and light trucks with gasoline engines since 1980-1981. Most vehicles built before the mid-90’s have one or two oxygen sensors (two were used on selected V6 and V8 engines starting in the late 1980s). Oxygen sensors are normally located in the exhaust system before the catalytic converter to measure exhaust emissions as they come from the engine combustion chambers. In 1996 with the federal mandated use of on-board-diagnostic systems (OBDII), vehicles also require additional oxygen sensors after the catalytic converter, to ensure that the converter is operating properly.
Click here to see charts showing how the emissions change based on the air/fuel ratio with and without a catalytic converter.
I have a Bosch original equipment 4-wire sensor. What do the wire colors mean? All Bosch 4-wire sensors have a black wire for the signal, a gray wire for ground and 2 white wires for the heater. Note: the wire colors and functions on non-Bosch sensors are not necessarily the same as on Bosch sensors.
How do you install a Bosch Universal Heated Sensor? Click here for instructions.
Does Bosch test all oxygen sensors manufactured to ensure their quality? Yes! Bosch tests all oxygen sensors it manufactures by subjecting each of them to a 1,000 degree Celsius functional test and a leak test to ensure environmental and functional robustness. The 1,000 degree Celsius test darkens the outer protection tube, but ensures 100% functional sensor performance.
What is a universal heated oxygen sensor? Bosch has created an aftermarket universal heated oxygen sensor program. These sensors meet OEM operating requirements and have a patented connector system easing installation. This connector system has been proven to be watertight, protect against contamination, and withstand the effects of extreme temperatures and engine vibration. Currently Bosch offers 12 different 4 wire sensors and 2 different 3 wire sensors to provide the closest match to OEM sensor performance.
Click here for a diagram of the connector system, featuring special high temperature Posi-Lock® connectors.
Why should I change a damaged Oxygen Sensor? According to a study conducted by Sierra Research Inc. in 1996, worn-out oxygen sensors are the “single greatest source of excessive emissions for fuel-injected vehicles” and the second most significant cause of high emissions in carburetor engines. The U.S. Environmental Protection Agency (EPA) and the California Air Resources 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 levels of hydrocarbons or carbon monoxide. Testing the oxygen sensors according to the vehicle manufacturer’s service procedures and replacing a sluggish or worn-out oxygen sensor can improve fuel economy from 10% to 15% (on pre-OBDII vehicles) and pay for itself in a year in fuel savings alone, while restoring your vehicle’s emissions to proper levels. And it can reduce the chance of an overly rich fuel mixture damaging the vehicle’s catalytic converter.