Bad Oxygen Sensor Symptoms

I'm going to go a bit of a different route on this. Any technicians out
there that have had an oxygen sensor go bad? What are the usual symptoms?
For the record, I have an 89 Civic LX, 1.5L engine, auto. transmission with
229K miles. The computer has set a Code 1 which the Service Manual says is
oxygen related. I don't want to start hanging parts. Thought I'd ask if
anyone has run across this before.

Thanks.

 

wolfer,
I found this on the web somewhere a long time ago, I did not write it but
I wish I could put it as clearly as Rick did. I hope this helps,
Scott.

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Oxygen Sensor InformationOxygen Sensor Information
In response to several requests for more information about Oxygen (O2)
sensors, perhaps the following information will help. Comment:
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.
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.
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 kill 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.
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.



Rick Kirchhof Austin, Texas | Experience is what you
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