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Premium Member
9,213 Posts
Discussion Starter · #1 · (Edited)
O2 Oxygen Sensor Information

I hope this will be a sticky post...

Premium Member
9,213 Posts
Discussion Starter · #2 ·
Written by Rick Kirchoff ([email protected]). Edited to html by Kyle Hamar

In response to several requests for more information about Oxygen (O2)
sensors, perhaps the following information will help.


These procedures are only for self powered conventional sensors.
Some very new cars are using a different style sensor that is
powered. *Many* Oxygen sensors are replaced that are good to
excellent. *Many* people don't know how to test them. They
routinely last 50,000 or more miles, and if the engine is in good
shape, can last the life of the car.

Premium Member
9,213 Posts
Discussion Starter · #3 ·
What does the O2 sensor do?

It is the primary measurement device for the fuel control computer
in your car to know if the engine is too rich or too lean. The
O2 sensor is active anytime it is hot enough, but the computer
only uses this information in the closed loop mode. Closed loop
is the operating mode where all engine control sensors including
the Oxygen sensor are used to get best fuel economy, lowest
emissions, and good power.

Premium Member
9,213 Posts
Discussion Starter · #4 ·
Should the O2 sensor be replaced when the sensor light comes on in
your car?

Probably not, but you should test it to make sure it is alive and
well. This assumes that the light you see is simply an emissions
service reminder light and not a failure light. A reminder light
is triggered by a mileage event (20-40,000 miles usually) or
something like 2000 key start cycles. EGR dash lights usually fall
into the reminder category. Consult your owners manual, auto repair
manual, dealer, or repair shop for help on what your light means.

Premium Member
9,213 Posts
Discussion Starter · #5 ·
How do I know if my O2 sensor may be bad?

If your car has lost several miles per gallon of fuel economy and
the usual tune up steps do not improve it. This *is not* a
pointer to O2 failure, it just brings up the possibility. Vacuum
leaks and ignition problems are common fuel economy destroyers.
As mentioned by others, the on board computer may also set one of
several failure "codes". If the computer has issued a code
pertaining to the O2 sensor, the sensor and it's wiring should
be tested. Usually when the sensor is bad, the engine will show
some loss of power, and will not seem to respond quickly.

Premium Member
9,213 Posts
Discussion Starter · #6 ·
Will testing the O2 sensor hurt it?

Almost always, the answer is no. You must be careful to not
*apply* voltage to the sensor, but measuring it's output voltage
is not harmful. As noted by other posters, a cheap voltmeter
will not be accurate, but will cause no damage. This is *not*
true if you try to measure the resistance of the sensor.
Resistance measurements send voltage into a circuit and check the
amount returning.

Premium Member
9,213 Posts
Discussion Starter · #7 ·
How does an O2 sensor work?

An Oxygen sensor is a chemical generator. It is constantly making
a comparison between the Oxygen inside the exhaust manifold and air
outside the engine. If this comparison shows little or no
Oxygen in the exhaust manifold, a voltage is generated. The
output of the sensor is usually between 0 and 1.1 volts. All
spark combustion engines need the proper air fuel ratio to
operate correctly. For gasoline this is 14.7 parts of air to one
part of fuel. When the engine has more fuel than needed, all
available Oxygen is consumed in the cylinder and gasses leaving
through the exhaust contain almost no Oxygen. This sends out a
voltage greater than 0.45 volts. If the engine is running lean,
all fuel is burned, and the extra Oxygen leaves the cylinder and
flows into the exhaust. In this case, the sensor voltage goes
lower than 0.45 volts. Usually the output range seen seen is
0.2 to 0.7 volts.

The sensor does not begin to generate it's full output until it
reaches about 600 degrees F. Prior to this time the sensor is
not conductive. It is as if the circuit between the sensor and
computer is not complete. The mid point is about 0.45 volts.
This is neither rich nor lean. A fully warm O2 sensor *will not
spend any time at 0.45 volts*. In many cars, the computer sends
out a bias voltage of 0.45 through the O2 sensor wire. If the
sensor is not warm, or if the circuit is not complete, the computer
picks up a steady 0.45 volts. Since the computer knows this is
an "illegal" value, it judges the sensor to not be ready. It
remains in open loop operation, and uses all sensors except the
O2 to determine fuel delivery. Any time an engine is operated
in open loop, it runs somewhat rich and makes more exhaust
emissions. This translates into lost power, poor fuel economy
and air pollution.

The O2 sensor is constantly in a state of transition between high
and low voltage. Manfucturers call this crossing of the 0.45
volt mark O2 cross counts. The higher the number of O2 cross
counts, the better the sensor and other parts of the computer
control system are working. It is important to remember that the
O2 sensor is comparing the amount of Oxygen inside and outside
the engine. If the outside of the sensor should become blocked,
or coated with oil, sound insulation, undercoating or antifreeze,
(among other things), this comparison is not possible.

Premium Member
9,213 Posts
Discussion Starter · #8 ·
How can I test my O2 sensor?

They can be tested both in the car and out. If you have a high
impedence volt meter, the procedure is fairly simple. It will
help you to have some background on the way the sensor does
it's job. Read how does an O2 sensor work first.

Premium Member
9,213 Posts
Discussion Starter · #9 ·
Testing O2 sensors that are installed

The engine must first be fully warm. If you have a defective
thermostat, this test may not be possible due to a minimum
temperature required for closed loop operation. Attach the
positive lead of a high impedence DC voltmeter to the Oxygen
sensor output wire. This wire should remain attached to the
computer. You will have to back probe the connection or use
a jumper wire to get access. The negative lead should be
attached to a good clean ground on the engine block or
accessory bracket. Cheap voltmeters will not give accurate
results because they load down the circuit and absorb the
voltage that they are attempting to measure. A acceptable
value is 1,000,000 ohms/volt or more on the DC voltage.
Most (if not all) digital voltmeters meet this need. Few
(if any) non-powered analog (needle style) voltmeters do.
Check the specs for your meter to find out. Set your meter
to look for 1 volt DC. Many late model cars use a heated
O2 sensor. These have either two or three wires instead of
one. Heated sensors will have 12 volts on one lead, ground
on the other, and the sensor signal on the third. If you have
two or three wires, use a 15 or higher volt scale on the meter
until you know which is the sensor output wire.

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 your connections. Next, check your leads to make sure
you won't wrap up any wires in the belts, etc. then start the
engine. You should run the engine above 2000 rpm for two
minutes to warm the O2 sensor and try to get into closed loop.
Closed loop operation is indicated by the sensor showing several
cross counts per second. It may help to rev the engine between
idle and about 3000 rpm several times. The computer recognizes
the sensor as hot and active once there are several cross counts.

You 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 (> 0.45) near 0.45 or steady low (< 0.45).
If the voltage is near the middle, you may not be hot yet. Run
the engine above 2000 rpm again. If the reading is steady low,
add richness by partially closing the choke or adding some propane
through the air intake. Be very careful if you work with any
extra gasoline, you can easily be burned or have an explosion.
If the voltage now rises above 0.7 to 0.9, and you can change it
at will by changing the extra fuel, the O2 sensor is usually good.

If the voltage is steady high, create a vacuum leak. Try pulling
the PCV valve out of it's hose 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, it
was dead anyway, no loss. In either case, fix the cause of the
rich mixture and retest. If you don't, the new sensor will

Premium Member
9,213 Posts
Discussion Starter · #10 ·
Testing O2 sensors on the workbench.

Use a high impedence DC voltmeter as above. Clamp the sensor in
a vice, or use a plier or vice-grip to hold it. Clamp your
negative voltmeter lead to the case, and the positive to the
output wire. Use a propane torch set to high and the inner blue
flame tip to heat the fluted or perforated area of the sensor.
You should see a DC voltage of at least 0.6 within 20 seconds.
If not, most likely cause is open circuit internally or lead
fouling. If OK so far, remove from flame. You should see a
drop to under 0.1 volt within 4 seconds. If not likely silicone
fouled. If still OK, heat for two full minutes and watch for
drops in voltage. Sometimes, the internal connections will open
up under heat. This is the same a loose wire and is a failure.
If the sensor is OK at this point, and will switch from high to
low quickly as you move the flame, the sensor is good. Bear in
mind that good or bad is relative, with port fuel injection
needing faster information than carbureted systems.

ANY O2 sensor that will generate 0.9 volts or more when heated,
show 0.1 volts or less within one second of flame removal, AND
pass the two minute heat test is good regardless of age. When
replacing a sensor, don't miss the opportunity to use the test
above on the replacement. This will calibrate your evaluation
skills and save you money in the future. There is almost always
*no* benefit in replacing an oxygen sensor that will pass the
test in the first line of this paragraph.

Premium Member
22,915 Posts
May I ask why? I don't see hardly anyone actually, I see no one asking what a O2 sensor is or how it works and thats ussually why they sticky threads because there is an abundance of questions on it.

Premium Member
9,213 Posts
Discussion Starter · #12 ·
Its just a FYI, if nobody wants to know then it will fall to the bottom like the rest :-D

251 Posts
If it falls to the bottom of the thread list it should be fine. It is useful information that will show up when searched for.

Premium Member
699 Posts
People seem to fail to realise that the O2 is the best sensor to test yr engine performance from. Yeah it is used by the ecu to determine short term fuel trim, but it also the best sensor to determine driveability problems. With a good quality graphing meter or scope, the O2 can show you ignition/fuel misfire, injector problems, vacuum leaks, egr performance, turbulence in the exhaust from the valve train. And all the info can be correlated to other primary sensors to determine engine performance.

1,791 Posts
You guys got me wondering if my '91na O2 sensor was okay? (Gas mileage seems about 10% lower than what I'd expect... 27 instead of over 30 mpg at 70 mph.) Following the BGB, the O2 sensor test doesn't show any voltage fluctuations when the engine is warm; thus it seems bad. But how do I install a standard 4-wire Toyota or OEM sensor when the harness doesn't have necessary wires? Who knows what the previous owner did, but it looks like he replaced it with a single wire sensor. (see photo) This O2 sensor and connector don't look standard, do they?:

(Later edit: it turned out to be a one-wire Toyota 02 sensor after all).

(1) Are there any significant disadvantages to using a 1-wire sensor in a MR2 that is usually driven long distances and thus warms up for much of its miles? (start-up pollution can't be all that much of a concern since this isn't a stop-and-go kind of car).
(2) Is there a way to retrofit the needed wires to install a 4-wire O2 sensor without having to trace the wiring harness all the way back to the ECU?

With the great advice I read above, I found a way to fix it without replacing it. I held the 02 sensor's tip in the blue flame tip of a handheld propane torch for 2 minutes while measuring the voltage between the body and the sensor's wire. For most of that time it measured 0 V. Then, as the accumulated exhaust deposits burnt away on the tip, the voltage increased.... slowly at first.. and then up to 0.9 V. Now it functions okay, dropping to less than 0.1 V in 1 - 2 seconds after the flame is removed. That's a cheap solution possible for some 02 sensors that are merely 'clogged.' Thanks, guys!

Premium Member
9,213 Posts
Discussion Starter · #19 ·
glad this this helped someone ;-)
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