oxygen sensors part one of two

[Chongo]

What does the O2 sensor do?

Made of a cobalt monoxide ceramic sensing element having an
electrical resistive path which figures a stoichiometric
value Or can be made of partially stabilized zirconia on one face of which is formed a measuring electrode exposed to a gas to be measured and on the other face of which a standard electrode exposed to a standard gas. An oxygen concentration detecting device provided with a protecting layer covers the measuring electrode formed on the oxygen concentration detecting device on the face exposed to the gas to be measured, the protecting layer being made of a partially stabilized sintered material of a partially stabilized zirconia material having a specific surface area of not less than 3 m2 /g and not more than 6.4 m2 /g to which a divalent or trivalent metal oxide, such as yttria, is added.




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.

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.

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.

What will damage my O2 sensor?

Home or professional auto repairs that have used silicone gasket
sealer that is not specifically labeled "Oxygen sensor safe",
"Sensor safe", or something similar, if used in an area that
is connected to the crankcase. This includes valve covers, oil
pan, or nearly any other gasket or seal that controls engine oil.
Leaded fuel will ruin the O2 sensor in a short time. If a car is
running rich over a long period, the sensor may become plugged up
or even destroyed. Just shorting out the sensor output wire will
not usually hurt the sensor. This simply grounds the output
voltage to zero. Once the wiring is repaired, the circuit
operates normally. Undercoating, antifreeze or oil on the
*outside* surface of the sensor can **** it. See how does an
Oxygen sensor work.

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.

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.

 

[Chongo]

oxygen sensors part 2 of 2

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.

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
fail.

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.

I hope this helps, feel free to ask any questions.............

Chongo.............

 

[gpt]

O2 sensor and valve cover gasket replacement

Interesting comment regarding gasket sealant material affecting the O2 sensors. I have a 2003 Disco II that went in for warranty work at 74,800 miles. Dealership replaced valve cover gaskets that were leaking oil both sides. They also replaced the gas cap since it would fail to seal giving me a Service Engine Light. 1st cap was replaced at 49,000 miles due to same problem. Drove from the dealership and had Service Engine Light come on within 100 miles or less. Thought it was the gas cap again. Light did not go off. Got code P0150 for O2 sensor fault. Replaced spark plugs, probably original Champions, with Autolite Platinums. Noticed my mileage dropped off on the highway from roughly 17 to 18 mpg to about 12 or 13 mpg, City driving dropped too! Original plugs were pretty well shot so it was time. Just thought it was an interesting coincidence that the O2 fault came right after the valve cover replacement. On top of it, the valve covers were seeping again within 1500 miles. Dealership said they had some pooching in the gaskets and would replace them again. Anybody have comment on Autolite plugs?

 

[Chongo]

O2 sensor and valve cover gasket replacement

Interesting comment regarding gasket sealant material affecting the O2 sensors. I have a 2003 Disco II that went in for warranty work at 74,800 miles. Dealership replaced valve cover gaskets that were leaking oil both sides. They also replaced the gas cap since it would fail to seal giving me a Service Engine Light. 1st cap was replaced at 49,000 miles due to same problem. Drove from the dealership and had Service Engine Light come on within 100 miles or less. Thought it was the gas cap again. Light did not go off. Got code P0150 for O2 sensor fault. Replaced spark plugs, probably original Champions, with Autolite Platinums. Noticed my mileage dropped off on the highway from roughly 17 to 18 mpg to about 12 or 13 mpg, City driving dropped too! Original plugs were pretty well shot so it was time. Just thought it was an interesting coincidence that the O2 fault came right after the valve cover replacement. On top of it, the valve covers were seeping again within 1500 miles. Dealership said they had some pooching in the gaskets and would replace them again. Anybody have comment on Autolite plugs?


Hello GPT:

Some sealants emit ammonia gasses for a certain length of time. While this occurs in the crank case areas, they are drawn in by the crankcase breather system, and inter into the combustion chamber. This produces extremely high acetic and caustic gases. These will attack most metallic surfaces, and will almost immediately, compromise the integrity of the oxygen sensors, reversing the chemical / electrical design of the sensor. This is usually caused by using silicones that have ammonia in them. Usually the oxygen sensors fail in under 10 hours of running time. This is why it is so important to use sensor safe sealants, they have no ammonia in them.

As far as the plugs go, which autolites did you use? Your mileage problem is most likely associated with the gasket materials, and the compromised oxygen sensors than anything else, with the given data.


You may find some important information here on the ignition system:

http://www.landroverworld.org/forums/showthread.php?t=12020


This should tell you what you want to know about plugs:


http://www.landroverworld.org/forums/showthread.php?t=11849


Let me know if I can help any further

Good luck ...............Chongo

 

[gpt]

Appreciate the info. Interesting thing happened this morning. Per earlier notation, I replaced driver side front O2, reset, drove 20 less miles and service enginer light came on. Reset again. Came on again 3 miles. Been on for a week. This morning service engine light off. Fingers are crossed. Will have the OBDII checked after 40 miles to see if it is clear. I had put Lucas in at a fill up and have run it near empty and have added more premium fuel. Got any ideas why the light would go off on it's own?

 

[gpt]

Spoke to soon. Light went back on. Will replace other sensor.

 

[wolf]

Hi Chongo;

Mucho gracias for even more great info on sensors!

Wolf

 

[Mark]

Pain

As much as it pains the wallet, replace all four of your O2 sensors and be done with it. I replaced my a year and a half ago and have never regretted the money spent. The best prices were from oxygensensors.com.

The installation was also a pain, so get help for the fron two.