O2 Sensor testing info

[am018]

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Attached is the info on O2 sensors from the defunct  David Kirchoff Web
site.

This has been OCR scanned in and may have errors


Andy

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This info was taken from David Kirchoffs web page which is no longer avai=
lable, as it was OCR scanned from a well worn print off some errors may e=
xist  in this version of the text.
All credits to the author David Kirchoff


Oxygen Sensor Information
In response to several requests for more information about Oxygen (02) se=
nsors, perhaps the following information will help. Comment:
 =

These procedures are only for self powered conventional sensors. Some ver=
y new cars are using a different style sensor that is powered. *Many* Oxy=
gen sensors are replaced that are good to excellent. *Many* people don't =
know how to test them. =

O2 Sensors routinely last 50,000 or more miles, and if the engine is in g=
ood shape, can last the life of the car.
 =

What does the 02 sensor do?
It is the primary measurement device for the fuel control computer in you=
r car to know if the engine is too rich or too lean. The 02 sensor is act=
ive 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 eng=
ine control sensors including the Oxygen sensor are used to get best fuel=
 economy, lowest emissions, and good power.
 =

Should the 02 sensor be replaced when the sensor light comes on in your c=
ar?
Probably not, but you should test it to make sure it is alive and well. T=
his assumes that the light you see is simply an emissions service reminde=
r light and not a failure light. A reminder light is triggered by a milea=
ge event (20-40,000 miles usually) or something like 2000 key start cycle=
s. EGR dash lights usually fall into the reminder category. Consult your =
owners manual, auto repair manual, dealer, or repair shop for help on wha=
t your light means.
 =

How do I know if my 02 sensor may be bad?
If your car has lost several miles per gallon of fuel economy and the usu=
al tune up steps do not improve it. This is * not* a pointer to 02 failur=
e, 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 02 sensor, the sensor and it's wiring sh=
ould 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 02 sensor?
Home or professional auto repairs that have used silicone gasket sealer t=
hat is not specifically labelled "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 02 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 wi=
ll not usually hurt the sensor (!!!??). This simply grounds the output vo=
ltage to zero. Once the wiring is repaired, the circuit operates normally=
=2E Undercoating, antifreeze or oil on the *outside* surface of the senso=
r can kill it. See how does an Oxygen sensor work.
 =

 =

Will testing the 02 sensor hurt it?
Almost always, the answer is no. You must be careful to not *apply* volta=
ge to the sensor, but measuring it's output voltage is not harmful. As no=
ted by other posters, a cheap voltmeter will not be accurate, but will ca=
use no damage. This is *not* true if you try to measure the resistance of=
 the sensor. =

Resistance measurements put a voltage across circuit and measure the resi=
stance from the resulting current.


How does a 02 sensor work?

An Oxygen Sensor is a chemical generator. It is constantly making a compa=
rison between oxygen content of the gas inside the exhaust manifold and a=
ir outside the engine. If this comparison shows little or oxygen in  the =
exhaust manifold, a voltage is generated. The output of the sensor is usu=
ally between 0 and 1.1 volts. All spark combustion engines need the prope=
r air fuel ratio to operate correctly. For petrol this is 14.7 parts of a=
ir to one part of fuel. When the engine has more fuel than needed, all av=
ailable Oxygen is consumed in the cylinder and gasses leaving through the=
 exhaust will 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 t=
he extra Oxygen leaves the cylinder and flows into the exhaust. In this c=
ase, the sensor voltage goes lower than 0.45 volts. Usually the output ra=
nge seen is 0.2 to 0.7 volts. The sensor does not begin to generate its f=
ull output until it reaches about 600 degrees F. Prior to this time the s=
ensor is not conductive. It is as if the circuit between the sensor and c=
omputer is not complete. The mid point is about 0.45 volts. This is neith=
er rich nor lean. Note that a fully warm O2 sensor ***will not spend any =
time at 0.45 volts even if the fuel injection system is working perfectly=
=2E In many cars, the computer sends out a bias voltage of 0.45 through t=
he O2 sensor wire. If the sensor is not warm, or if the circuit is not co=
mplete, the computer picks up a steady 0.45 volts. Since the computer kno=
ws this is an "illegal" value, it judges the sensor to not be ready. It r=
emains in open loop operation, W and uses all sensors except the O2 to de=
termine fuel delivery. =


When  an engine is operated in open loop, it runs somewhat rich and makes=
 more exhaust emissions. This translates into lost power, poor fuel econo=
my and air pollution. The O2 sensor is constantly in a state of transitio=
n between high and low voltage. Manufacturers call this crossing of the 0=
=2E45 volt mark 02 cross counts. The higher the number of O2 cross counts=
, the better the sensor and other parts of the computer control Ny,1tem a=
re working. It important to remember that the O2 sensor is comparing the =
amount of Oxygen  In exhaust gas with the atmosphere  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.

How can I test my 02 sensor?

They can be tested both in the car and out. If you have a high impedance =
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 sens=
or work first.

Testing 02 sensors that are installed

The engine must first be fully warmed through  to a minimum temperature r=
equired for closed loop operation. Attach the positive lead of a high imp=
edance DC voltmeter to the Oxygen sensor output wire. This wire should re=
main attached to the ECU computer. You will have to back probe the connec=
tion or use a jumper wire to get access. The negative lead should be atta=
ched to a good clean ground on the engine block or the case of the ECU. C=
heap voltmeters will not give accurate results because they load down the=
 circuit and absorb the voltage that they are attempting to measure. A ac=
ceptable value is 1,000,000 ohms/volt or more on the DC voltage. Generall=
y  digital multimeters/voltmeters can safely be used but ordinary analogu=
e voltmeters/multimeters  could both damage the sensor and give inaccurat=
e results.(if any) non-powered analogue meters do. =



Most  late model cars use a heated 02 sensor. These have either two or th=
ree 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 mak=
e sure you won't wrap up any wires in the belts, etc. then start the engi=
ne. You should run the engine above 2000 rpm for two minutes to warm the =
O2 sensor and try to get into closed loop. The sensor showing several cro=
ss counts per second indicates closed loop operation. It may help to rev =
the
engine between idle and about 3000 rpm several times. The computer recogn=
ises the sensor as hot and active once there are several cross counts. Yo=
u are looking for voltage to go above and below 0.45 volts. If you see le=
ss than 0.2 and more than 0.7 volts and the value changes rapidly, you ar=
e 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 stead=
y 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 gasol=
ine, 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 ext=
ra fuel, the 02 sensor is usually good. If the voltage is steady high, cr=
eate a vacuum leak. Try pulling the PCV valve out of it's hose and lettin=
g 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 yo=
u are not able to make a change either way, stop the engine, unhook the s=
ensor 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 connectio=
n, 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 carb=
on 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 might =
clean it off 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 02 sensors on the workbench.

Use a high impedance DC voltmeter as above. Clamp the sensor in a vice, o=
r use a pliers or vice-grips to hold it. Clamp your negative voltmeter le=
ad to the case and the positive to the output wire of the sensor.
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 a=
t 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 s=
ee a drop to under 0.1 volt within 4 seconds. If not likely silicone foul=
ed. If still OK, heat for two full minutes and watch for drops in voltage=
=2E 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 sen=
sor is good. Bear in mind that good or bad is relative, with port fuel in=
jection needing faster information than carburettor systems. ANY 02 senso=
r that will generate 0.9 volts or more when heated, and show 0.1 volts or=
 less within one second of flame removal, AND pass the two minute heat te=
st  is good regardless of age. When replacing a sensor, don't miss the op=
portunity 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=
=2E
	-.-	-.-.-	-.-	-.-	.................,,....... =

	Rick Kirchhof   Austin, Texas	I Experience is what you
	Domain: rick at posms.cactus.org	I get when you don't
	Bang path: . .. !cs.utexas.edu!peyote!~)~~~ms!rick	I get what you want.

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