The exhaust gas oxygen sensor (EGO or O2S), or Lambda sensor as the
Europeans call it, is the key sensor in the engine fuel control feedback
loop. The engine computer uses the O2 sensor's input to balance the fuel
mixture, leaning the mixture when the sensor reads rich and richening
the mixture when the sensor reads lean. By switching the air/fuel
mixture back and forth, the average mixture is properly balanced for
The oxygen sensor reacts to unburned oxygen in the exhaust. When the
fuel mixture is rich (an air/fuel ratio less than 14.7:1), there is
little unburned oxygen in the exhaust. This causes the O2 sensor's output
voltage to rise up to 600 to 1000 mv. When the fuel mixture
is lean (an air/fuel ratio greater than 14.7:1), there is more
unburned oxygen in the exhaust. This causes the O2 sensor's output
voltage to drop to 300 mv or less.
RICH MIXTURE (A/F over 14.7:1) = High O2 Voltage Output (600 to 1000 mv)
LEAN MIXTURE (A/F under 14.7:1) = Low O2 Voltage Output (300 mv or less)
When the air/fuel ratio is perfectly balanced at 14.7:1 (called the
"stoichiometric" ratio), the O2 sensor's output is about 450 mv. But
if the air fuel ratio changes either way (rich or lean), the O2
sensor's output voltage will quickly change up or down. So in effect,
the O2 sensor acts like a rich-lean indicator switch for the feedback
fuel control system.
BASIC OXYGEN SENSOR OPERATION
Most oxygen sensors are designed to act like a solid-state galvanic
battery and generate a voltage signal. This occurs because the
"Nernst effect" allows the zirconium dioxide ceramic sensor element
to become electrically conductive at high temperature, typically 617
to 662 degrees F (325 to 350 degrees C). The ceramic is slightly
porous so oxygen can pass through it, and it has a thin coating of
platinum on both sides that serve as electrodes. One side of the
ceramic sensor element is exposed to oxygen in the exhaust, and the
other side is exposed to oxygen in the atmosphere (there is either
a small vent in the sensor shell or air is allowed to enter through
the wiring connector).
When conditions are right (high temperature and different concentrations
of oxygen on opposite sides of the sensor element), the Nernst effect
creates a voltage across the ceramic between the platinum electrodes.
This generates a small output voltage that varies with the engine's
Low oxygen levels create a greater difference across the sensing
element, which causes the O2 sensor's output voltage to rise. Higher
oxygen levels reduce the difference across the sensing element and
cause the O2 sensor's output voltage to drop.
What's interesting about O2 sensors is that the voltage output switches
abruptly any time the air/fuel mixture goes rich or lean -- which
is most of the time! When an engine is running in "closed loop"
(meaning the computer is using the O2 sensor signal to regulate the
air/fuel ratio), the fuel mixture is constantly changing from rich to
lean and back again instead of running at a fixed or constant ratio.
This is necessary to maintain an overall balanced fuel mixture, and for
the catalytic converter to work properly.
Every time the air/fuel mixture goes rich, the O2 sensor's output
suddenly jumps up to 600 to 1000 mv. And every time the air/fuel
mixture goes lean, the O2 sensor's output quickly drops to 300 mv or
less. This rapid switching back and forth in the O2 sensor's output
voltage is what allows the engine computer to add or subtract fuel
to achieve a balanced fuel mixture. By averaging the swings back and
forth from rich to lean and adding or subtracting fuel as needed each
time, the overall ratio is kept near 14.7:1.
The speed at which an O2 sensor switches back and forth will vary with
the type of fuel delivery system on the engine. The slowest switching
occurs in older feedback carburetor systems that typically switch one
every second at 2,500 rpm. Throttle body injection systems are somewhat
faster, switching two to three times a second at 2,500 rpm. The fastest
switching occurs in multipoint injection systems, which may switch five
to seven times a second at 2,500 rpm.
The important point here is that the O2 sensor must react quickly to
changing fuel conditions in order for the computer to maintain the
best air/fuel ratio. As sensors age, they may become sluggish and
respond too slowly to changes in the air/fuel ratio. This can carbon
monoxide (CO) emissions and fuel consumption to rise.
DOWNSTREAM O2 SENSORS
On OBD2 equipped vehicles (some 1994 & 1995 models, and all 1996 and
newer cars and light trucks), a "downstream" O2 sensor is used to
monitor the operating efficiency of the catalytic converter. The
downstream O2 sensor works the same as the "upstream" O2 sensor that
is ahead of the converter, but the computer uses its input differently.
If the converter is doing its job efficiently, there will be little
unburned oxygen left in the exhaust. This should cause the downstream
O2 sensor to produce a steady output voltage instead of switching back
and forth like the upstream O2 sensor. If the downstream O2 sensor is
switching like the upstream O2 sensor, it means the converter isn't
On some applications (Ford and Chrysler), the engine computer also
uses the downstream O2 sensor signal for long term fuel trim
adjustments to the air/fuel ratio.
DIFFERENT TYPES OF O2 SENSORS
There are five basic types of O2 sensors. It's important to understand
the differences because replacement sensors must be the same basic
type as the original.
UNHEATED THIMBLE-TYPE O2 SENSORS
The unheated thimble-type O2 sensors are the oldest design, and have
been around since 1976 when Bosch introduced the first O2 sensors
for feedback fuel control on automotive engines. The zirconia ceramic
element is shaped like a long thimble and is located inside a protective
metal tube that extends into the exhaust manifold. Slots or holes in
the tip of the metal tube allow the hot exhaust gases to come into
contact with the ceramic thimble inside the tip of the sensor.
Reference air for the inside of the ceramic thimble may be provided
by a small vent hole in the sensor shell or through the wiring
connector. This type of sensor typically has a single wire connector,
though some have two wires.
This type of sensor produces a voltage signal that switches back and
forth as the air/fuel mixture changes.
Unheated O2 sensors can take up to several minutes to generate a
signal after a cold start because it they rely solely on the heat
from the exhaust to reach normal operating temperature. Consequently,
an unheated sensor may cool off at idle and stop producing a signal
causing the engine control system to revert back to "open loop"
operation (fixed air/fuel ratio setting).
HEATED THIMBLE-TYPE O2 SENSORS
This type of sensor was first introduced by Bosch in 1982, and is
used on 99 percent of vehicles through 1998. A heated O2 sensor has
the same type of zirconia ceramic thimble element as an unheated
sensors, but inside the thimble is a special resistance heating
element that brings the sensor up to operating temperature much
more quickly (in 30 to 60 seconds). The heater requires a separate
electrical circuit to supply voltage, so one way to identify a
heated sensor is to look for extra wires (3 or 4 wire connectors).
The heater allows the sensor to reach operating temperature more
quickly so the engine control system can go into closed loop sooner
to reduce cold start emissions. It also prevents the sensor from
cooling off at idle.
HEATED TITANIA-TYPE O2 SENSORS
Titania sensors use a different type of ceramic and produce a
different kind of signal than zirconia type sensors. Instead of
generating a voltage signal that changes with the air/fuel ratio,
the sensor's resistance changes. The resistance goes from low (less
than 1000 ohms) when the air/fuel ratio is rich to high (over 20,000
ohms) when the air/fuel ratio is lean. The switching point
occurs right at the ideal or stoichiometric air/fuel ratio. The
engine computer supplies a base reference voltage (1 volt or 5 volts
depending on the application), and then reads the change in the
sensor return voltage as the sensor's resistance changes. A lean
mixture will produce a low voltage signal (the same as a heated or
unheated zirconia O2 sensor) while a rich mixture will produce a
high voltage signal.
Titania O2 sensors are only used on a few applications, including
1986-93 Nissan 3.0L trucks, 1991-94 Nissan 3.0L Maxima and 1991-94
Nissan 2.0L Sentra. A 5-volt titania O2 sensor is also used on
1987-1990 Jeep Cherokee, Wrangler and Eagle Summit.
On the Jeep 4.0L applications, the titania O2 sensor has a 5v reference
voltage and is configured to work just the opposite of a conventional
zirconia O2 sensor. The titania O2 sensor on the Jeep will read high
(5 volts) when the mixture is lean, and low (1 volt) when the mixture is
rich. The Jeep sensor can be checked with an ohmmeter by disconnecting
the sensor (engine off) and measuring its resistance. It should be
between 5 and 7 ohms. An infinite (open) reading would indicate a bad
HEATED PLANAR-TYPE O2 SENSORS
This new type of sensor design developed by Bosch in 1997 uses a
flat ceramic zirconia element rather than a thimble. It's called a
"planar" sensor because the sensor element is a flat strip of ceramic
only 1.5 mm thick. The electrodes, conductive layer of ceramic,
insulation and heater are all laminated together on a single strip.
The new design works the same as the thimble type zirconia sensors,
but the "thick-film" construction makes it smaller and lighter, and
more resistant to contamination. The new heater element also requires
less electrical power and brings the sensor up to operating temperature
in only 10 seconds.
Outside reference air is supplied by a small port in the center of
the ceramic strip through the external electrical connector (which has
a spade-type connector and four wires).
The first application for planar O2 sensors was the 1998 VW 2.0L Beetle.
For 1999, it was used on Cadillac Catera, Saturn 3.0L LS and VW 2.0L Jetta.
In 2000, it was used on all Audis except the A4 1.8L turbo and A6 2.8L,
California Dodge 2.0L Neon, Ford 4.0L & 5.0L Explorer, Ford 2.5L LEV
Ranger, Ford 3.8L Windstar, Mercedes-Benz 3.2L ML320 and 4.3L ML430,
Mercury 4.0L & 5.0L Mountaineer, Saab 2.0L & 2.3L, and all VW and Volvo
models. For 2001, Porsche 911 3.6L Turbo got the new planar O2 sensor
along with all Mercedes-Benz models except the SL500 and SL600. For 2002,
the list grew to include all Audi models, all Dodge Neons, Ford F-Series
trucks (4.2L, 4.6L & 5.4L), all Ford Ranger trucks, Mazda B-Series pickups
(2.5L, 3.0L & 4.0L), all Mercedes-Benz models, and Saturn 3.0L SUV.
By model year 2004, planar O2 sensors are expected to account for 30% of
all O2 sensor applications, and by model year 2008 for up to three-fourths
of all O2 sensors.
HEATED WIDEBAND 02 SENSORS
The newest O2 sensor technology allows the ability to actually measure
the air/fuel ratio directly for the first time. Instead of switching back
and forth like all previous sensor designs, the new wideband O2 sensors
produce a signal that is directly proportional to the air/fuel ratio.
Thus, this new type of O2 sensor is also called an "air/fuel ratio" sensor.
Wideband O2 sensors contain a "dual sensing element" that combines
the Nernst effect cell in the planar design with an additional
"oxygen pump" layer and "diffusion gap" on the same strip of
ceramic. This gives the sensor it's ability to accurately measure
oxygen levels in the exhaust, and thus the air/fuel ratio.
Wideband O2 sensors require a higher operating temperature, so the
built-in heater draws more amps to keep the sensor at 1200 to 1400
degrees F (700 to 800 degrees C).
There are two different types: one made by Denso (4-wire) for Toyota, Lexus
and Hondas, and another made by Bosch (5-wire) for U.S. and European cars.
The Denso wideband air.fuel sensors are found on 1997 California Toyota models,
and 1999 and newer Toyota, Lexus and Honda models. The Bosch wideband
air/fuel ratio sensors are used on 2002 Audi A4 & Quattro 1.8L,
2000-01 Cadillac Catera 3.0L, 2001-02 Porsche 911 3.6L, 2000-01 Subaru Legacy
& Outback 2.5L, 2000-02 Volkswagen Golf 1.8L, 2.0L & 2.6L, 2000-02 Volkswagen
Jetta 2.0L & 2.8L, 2002 Volkswagen Passat W8 4.0L and 1999-02 Volvo 2.3L,
2.4L & 2.8L engines.
Both types of wideband O2 sensors can read air/fuel ratios from extremely
rich (10:1) to straight air! Unlike a conventional O2 sensor that produces
a voltage that bounces back and forth from 0.1 to 0.9 volts to give a rich or
lean fuel mixture indication, the wideband sensor's output varies in
proportion to the exact air/fuel ratio.
The Denso wideband sensors produce a voltage signal that goes from a
low of about 2.4 volts when the air/fuel ratio is rich (10:1), up to
a high of 5.5 volts when the mixture is lean (22:1). At 14.7:1, the
Denso wideband O2 sensor produces about 3.3 volts.
If you hook up a scan tool to a late model Toyota with a wideband
sensor, you may not see the actual O2 sensor voltage. What you will see
is a "simulated" voltage reading between 0 and 1 volt. The actual voltage
output from the wideband sensor is much higher, but the computer is
calibrated to divide the sensor's actual output by 5 to comply with OBD II
regulations that require a display reading between 0 to 1 volts (these
regulations have since been revised to allow OEMs to display the actual
O2 sensor voltage regardless of the range).
The Bosch wideband O2 sensors produce a current signal rather than a
voltage. The sensor receives a reference voltage from the engine computer
and generates a signal current that varies according to the fuel mixture.
When the air/fuel mixture is perfectly balanced at 14.7:1, the Bosch
wideband sensor produces no output current.
When the air/fuel mixture is rich, the Bosch wideband O2 sensor produces a
"negative" current that goes from zero to about 2.0 milliamps as the
mixture gets richer. This requires a scope to view the sensor's wave pattern.
When the air/fuel mixture is lean, the Bosch wideband O2 sensor produces a
"positive" current that goes from zero up to 1.5 milliamps as the mixture
WIDEBAND O2 SENSOR DIAGNOSIS
Because of the internal circuitry used in wideband oxygen sensors, you can't
hook up a voltmeter to read a Bosch wideband sensor's output directly. You
can view the waveform on a scope, or use a scan tool and the vehicles onboard
diagnostics to check the sensor's output.
Your scan tool will display the actual air/fuel ratio. It can also show the
O2 sensor's response to changes that should cause a change in the air/fuel ratio.
Opening the throttle wide, for example, traditionally causes a sudden and brief
lean condition followed by a richer mixture as the computer compensates. But with
the new control strategies made possible with wideband O2 sensors, the air/fuel
ratio remains much more steady when the throttle is snapped open.
The diagnostic strategies for wideband O2 sensors vary from one vehicle
manufacturer to another, but as a rule you'll get an oxygen sensor code
if the sensor reads out of its normal range, if the readings don't make sense
to the computer (should indicate lean when lean conditions exist, etc.), or
if the heater circuit fails.
One thing to keep in mind about wideband O2 sensors is that they can be fooled
the same as a conventional oxygen sensor by air leaks between the exhaust manifold
and head, and by misfires that allow unburned oxygen to pass through into the
exhaust. Either will cause the sensor to indicate a false lean condition, which
in turn will cause the computer to make the engine run rich.
All types of O2 sensors can also be contaminated by silicone from coolant leaks
inside the engine (blown head gasket or cracked head), and by phosphorus from
burning oil (worn valve guides, rings & cylinders).