this article, we will discuss oxygen sensors, what they do and how to test
oxygen sensors. I will try to explain a little about them and answer a few
questions. Keep in mind that this article is not vehicle specific, but
applies to all cars in general. As always, if you are in doubt about
something, consult your owner’s manual or repair guide for vehicle
When should I replace the oxygen sensor?
oxygen sensors require replacement around 60 to 100K miles. You should
check the owner’s manual or repair guide for the recommended mileage for
your car. Most modern cars have a service light or gauge that lights up
when the preset mileage has been reached. This light is usually triggered
by a few different things, such as a mechanical mileage counter or by
counting a certain amount of turns of the key in the ignition. In these
cases, the light is merely a reminder to service the sensor. Be sure not
to confuse a service reminder light with an actually failure light. A
failure light is a light that only illuminates when the sensor actually
fails. Check the owner’s manual or repair guide if you are in any doubt
as to whether or not the light is a failure light or service light.
It is not always necessary to replace the sensor at the factory
recommended mileage however. These are usually just a service
recommendation. Naturally, parts do wear out and fail over time, so it’s
always a good idea to practice preventative maintenance on your car.
What are the symptoms of a bad oxygen
biggest indicator of a faulty oxygen sensor is a noticeable decrease in
fuel economy, along with a rich mixture. Now, this does not automatically
indicate that the sensor has failed. Be sure to check all vacuum hoses for
leaks as well as the ignition system, check the plugs, (are they fouled?)
check the rotor, distributor cap, points, spark plug leads, and condenser
(check all of these where applicable) Vacuum leaks and ignition problems
are notorious for causing fuel economy problems. It’s a good idea to
inspect and re-new the vacuum hoses every couple of years anyway. Other
symptoms of a faulty sensor are a loss of power, (particularly when
accelerating from a stand-still), overheating, and spark plug fouling,
both of these are due to an increased rich or lean running condition.
modern cars nowadays come with sort of a built in troubleshooting guide
that can help you diagnose if the sensor is bad. There is usually a port
or connector where you can plug in a computer and extract codes from the
fuel injection computer. In the case of most new cars, when an error
occurs, it generates a code and stores it. Using the computer, you can
extract the code from the fuel injection computer. This is invaluable in
determining not only a bad oxygen sensor, but all sorts of other problems
as well. In many cases, this can tell you if the sensor is bad, however if
it does not, keep reading, the steps below will go over how to test the
What can damage the oxygen sensor?
but not always, certain home or shop repairs can possibly damage an oxygen
sensor. Usually, the sensor is placed in the exhaust manifold very near
the bottom of the engine. It is in this area that you have components such
as oil pans, valve covers, and other panels and covers that are usually
sealed with a silicone-based sealer such as RTV. Some of these sealers
contain fumes that will damage the sensor. If you are working in the area
of the sensor, be sure to use a silicone-based sealer that is labeled
“Oxygen Sensor Safe” You can find this sealer at any auto parts store.
Other things that can destroy a sensor are anti-freeze/coolant, using
leaded fuel (even though it is hard to find these days), as well as a
sustained rich running engine. (This is usually the case when the oxygen
sensor fails) When the engine is running rich for an extended amount of
time, carbon will begin to build up on the inlets for the sensor, clog it,
and eventually burn it out.
it safe to test an oxygen sensor?
long as you are merely testing the voltage output, it is usually safe to
test the sensor. You do not want to apply any sort of voltage to the
sensor, this can fry the sensor. Also, you do not want to check resistance
between terminals as checking resistance means that you are sending
voltage into a circuit, and measuring the amount returning. This can fry
the sensor as well.
How does this thing work?
sensors are essentially chemical generators. They work by constantly
measuring the oxygen content inside the exhaust manifold and comparing it
to the air outside the engine. If this comparison shows little or no
oxygen in the exhaust manifold, a voltage is generated. This voltage is
then sent to the fuel injection computer, where it is received and based
on the voltage, the fuel injection computer makes the necessary
adjustments to change the overall fuel-air mixture of the engine. When the
oxygen sensor measures the correct mixture, the voltage drops and it sends
the appropriate signal to the computer to stop adjusting the mixture. This
is a non-stop exchange of signals between the sensor and the computer, and
it is constantly making adjustments depending on the needs of the engine.
When the sensor fails, it stops sending voltage to the fuel injection
computer, and usually the computer interprets this as “ok, we aren’t
getting a signal, so we better enrich this engine as much as we can”,
hence a rich running engine.
sensors usually put out a very small voltage when measuring oxygen
content. Typically, they only put out from 0 to 1.1 volts max.
internal combustion engines need a proper air-fuel ratio in order to run
correctly. Gasoline engines need to run a ratio of 14.7 parts of air to
one part of fuel. (Air cooled motors typically need to be slightly richer).
an engine has more fuel than it needs, all the oxygen in the cylinder head
is consumed and the resulting exhaust gases contain almost no oxygen. This
usually causes the sensor to generate a voltage of greater than 0.45
volts. If the engine is running too lean, all the fuel is burned, and the
extra oxygen remaining flows out into the exhaust manifold. The oxygen
sensor detects the extra oxygen and the voltage signal will drop to below
0.45 volts. .
0.45 volts is usually the voltage where the car is neither running rich
nor lean, this is when the car’s mixture is usually perfect.
sensors usually generate an output voltage between 0.2 to 0.7 volts.
However, Oxygen sensors doe not begin to generate full output voltage
until it reaches about 600 degrees F. Some oxygen sensors have a built-in
heating element that lets the sensor heat up quicker.
Keep in mind that most oxygen sensors work in an open loop until
they have reached a certain temperature. What this means is that until it
has heated up, the car will not use the oxygen sensor to measure the
mixture, but rather use all the other sensors on the car. The sensor will
generate a constant 0.45 volts until it has heated up.
Once the sensor reaches a certain temperature, it becomes a closed
loop and the sensor starts to generate variable voltage.
How do I test the sensor?
first step is to let the car warm up to operating temperature. You will
need a high-impedance DC voltmeter to measure the output voltage. It’s a
good idea to use a high quality or digital voltmeter. Analog voltmeters
usually are not sensitive enough to register the small voltages generated
by the oxygen sensor.
first step is to get the engine warmed up to operating temperature. This
insures that the oxygen sensor will generate voltage. Now, attach the
positive lead of the voltmeter to the oxygen sensor output wire.
wire should remain connected to the harness going to the computer, so you
may find it necessary to use a jumper or trim back the insulation so you
can attach the leads. Connect the negative lead to a good engine ground,
such as the engine block, or any bare metal on the vehicle’s chassis.
Now, set the voltmeter to look for 1 volt DC.
When you turn the key on, do not start the engine. You should see a
change in voltage on the meter in most late model cars. If not, check the
start the engine. In the case
of single wire sensors, you should run the engine above 2000 rpm for a few
minutes to heat up the O2 sensor and try to get into closed loop. The
sensor showing several cross counts per second indicates closed loop
operation. It may help to rev the engine between idle and about 3000 rpm
several times. The computer will recognize the sensor as hot and active
once there are several cross counts.
are looking for voltage to go above and below 0.45 volts. If you see less
than 0.2 and more than 0.7 volts and the value changes rapidly, you are
through, your sensor is good. If not, is it steady high, near 0.45 or
steady low? If the voltage is
near the middle, you may not be hot yet. Run the engine above 2000 rpm
again. If the voltage is steady high, create a vacuum leak. Try pulling
the PCV valve out of its hose
or remove the oil filler cap and letting air enter. You can also
use the power brake vacuum supply hose. If this drives the voltage to 0.2
to 0.3 or less and you can control it at will by opening and closing the
vacuum leak, the sensor is usually good. If you are not able to make a
change either way, stop the engine, unhook the sensor wire from the
computer harness, and reattach your voltmeter to the sensor output wire.
Repeat the rich and lean steps. If you can't get the sensor voltage to
change, and you have a good sensor and ground connection, try heating it
once more. Repeat the rich and lean steps. If still no voltage or fixed
voltage, you have a bad sensor. If you are not getting a voltage and the
car has been running rich lately, the sensor may be carbon fouled. It is
sometimes possible to clean a sensor in the car. Do this by unplugging the
sensor harness, warming up the engine, and creating a lean condition at
about 2000 rpm for 1 or 2 minutes. Create a big enough vacuum leak so that
the engine begins to slow down. The extra heat will clean it off if
possible. If not, the sensor is fried. In either case, fix the cause of
the rich mixture and retest. If you don't, the new sensor will fail.
My car has two (or
more) wires coming out of the sensor, which wire is the signal output?
Most modern cars
used a heated oxygen sensor. In this case, there will be two three or four
wires instead of one. What the heating element does is heat the oxygen
sensor up quicker than simply relying on the heat of the exhaust manifold.
If you have two,
three, or four wires, use a 15 or higher volt scale on the meter to test
each wire for 12 volts. You will need to determine which wire is the
signal output wire. Once you have determined which wire is the output
signal wire, test the sensor as shown above.
A heated oxygen
sensor with two wires is usually wired like this (pic
o2_sensor_diagram_2.jpg) One wire is 12 volts for the heating element. The
other wire is the signal output wire to the fuel injection computer.
In this case, the oxygen sensor casing is the ground.
a 3 wire oxygen sensor, it is wired like this, (pic
o2_sensor_diagram_3.jpg) one wire is 12 volts for the heating
element, one is ground for the heating element, and the last wire is the
signal output to the fuel injection computer.
with a 4-wire oxygen sensor, the output signal works in a constant loop,
essentially, the fuel injection computer sends a signal to the sensor, and
the sensor then sends the signal back to the computer. This is achieved by
having one wire carry the signal to the sensor, and then another wire
carries the signal back to the fuel injection computer. You also have a
wire carrying 12 volts to the heating element, and the last wire is the
ground for the heating element. (pic
o2_sensor_diagram_4 jpg) With a 4-wire sensor, you will need to
measure the voltage fluctuations between the two signal wires. To do this,
start the car and let it warm up. Next, disconnect the oxygen sensor and
measure the voltage between the wires. Now take off the oil filler cap to
simulate a rich running condition. Watch the voltage, if it begins to
fluctuate rapidly, then the sensor is good and you’re done. If it stays
the same, it’s probably fried.
Well, there you have it - it's
really not too difficult at all. If you would like
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