Oxygen Sensor Information

Written by Rick Kirchoff (rick_at_posms.cactus.org). Edited to html by Kyle Hamar

>From Terrill_Yuhas at smtpsc1.sc.pima.gov Fri Nov 4 12:42:39 1994

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.

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
Domain: rick_at_posms.cactus.org                   | get when you don't

Bang path: ...!cs.utexas.edu!peyote!posms!rick | get what you want.