Oxygen Sensor Testing

8/6/2019 Oxygen Sensor Testing

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1Oxygen Sensor TestingO2 probes generate a voltagebetween OV and 1V depending onthe O2 content of the exhaust gassurrounding the probe tipcompared to the O2 content of theambient air reaching the elementvia a vent in the sensor housing.With a "good" sensor and motor,the voltage reading will swingrapidly between 0V and 1V as theelectronics respond to inputs i.e.you will not have a steady reading.This combined with very low signalstrength, means measurementscan only realistically be madeusing a digital high impedancemultimeter with a bar graph, orusing specialised equipment.

Most sensors,including the oneswe are talking about, have aZirconia element that requires atip temp of around 300C+ toproduce a usable signal. This inturn means that probes without abuilt in heating element requirean exhaust gas temp at the probe

in excess of that produced at idlepower, and to get a valid readingusually a minimum of 2500 RPM isrequired. - When a heater is builtinto the probe, which as far as Ican tell is the case with all BMW

bikes, there will be 4 wiresconnected to it as opposed toeither 1 or 2. Measurements canthen be made at lower enginespeeds but they do need to beabove idle to stabilise results - say1500 RPM.With a definite system fault (and agood O2 sensor) an engine runninglean will cause the sensor toswing around a relatively lowvoltage (0.2V) and richness willcause the output voltage to swinghigh (0.8V) If no faults are presentand everything is operatingcorrectly the output will rapidlyswing between 0V and 1V. Whilstthe actual voltage measurementwill fluctuate too rapidly to read

consistently and is of no interest,the bar graph will rise and fallobservably as will the mean of theswing. Once you have seen thiseffect on a good sensor it is easilyrecognised so if you are into thistype of diagnostics, test your bikebefore a fault is suspected to get

experience.Testing - There is NO effective testavailable to the general workshopwith the probe off the vehicle butthe sensors are easy to test in situ.


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2First - The sensor is earthed andreferenced to earth via its outercase the exhaust pipe, engineblock and battery - this is true

even of 2 and 4 wire systems.Check for voltage drops and makesure the probe is secure and theearths are good. Make surevisually that the venting orifice isclear and externally the sensor isclean.Second - Get the motor up to temp,after a good highish RPM run, andestablish a fast idle so that theexhaust gas stream (and heater iffitted) is hot enough to allow thesensor to operate (above say 1500RPM). Probe the output(backprobing at the first plug up

from the sensor is a good spot)and make sure the voltage israpidly swinging as described.To test further, create lean andrich conditions:

y Remove an injector lead andsee that the voltage swingsin the lower 0 to 0.2V range.

y Clamp off the fuel returnline A LITTLE to raise railpressure and ensure thevoltage swing rises to the0.7 to 1V range.

About the only common faults arebad earths, dirt on the sensorbody, and age related failure whenthe output gets sluggish and slows

or even stops - this usually takesabout 70K miles or longeralthough failure will be almostimmediate if leaded petrol orsilicon based sealants have beenused where they can reach theprobe or combustion process. Asluggish sensor or one that fails torespond to rich and leanconditions instantly, has to bereplaced, they are not repairable.Now to the question ofcompatibility with aftermarketprobes. Never having had toreplace a BMW bike O2 sensor I

can only describe how I would goabout it!Obviously there has to be aphysical fit. Then there is thewiring configuration. Sensors comewith 1, 2, 3, 4, and 5 wires. As far asI know all BMW bikes come with afour wire probe and this will not

be specific to bikes. Two wires willbe to the element heater. Two willbe the O2 sensor circuit.Incidentally the wires from theprobe to the first connector willNOT reflect the proper harness


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3colours. All colours are per theharness beyond the first connectorwhich should only be a few inchesup from the sensor. If the

connector as well as the probedimensions are compatible itshould be safe to try, but easierstill is to check the suppliers crossreference chart. It may not containbike information but if the probefits, the plug matches, and isusable on BMW cars or other Boschsupplied systems then it should beOK.As far as I can tell all the bikesuse the same sensor with 4 wirescoloured as follows.K1100Gn/W & Br: Heater

Br & Y: Sensor (Y = signal)

K1200Gn/W &Gn/Br: HeaterY & Bk: Sensor (Y = signal)

Oxygen Sensor InformationIn response to several requests for

more information about Oxygen(O2) sensors, perhaps the followinginformation will help. Comment:These procedures are only for selfpowered conventional sensors.Some very new cars are using a

different style sensor that ispowered. *Many* Oxygen sensorsare replaced that are good toexcellent. *Many* people don't

know how to test them. Theyroutinely last 50,000 or moremiles, and if the engine is in goodshape, can last the life of the car.What does the O2 sensor do?It is the primary measurementdevice for the fuel controlcomputer in your car to know if theengine is too rich or too lean. TheO2 sensor is active anytime it ishot enough, but the computer onlyuses this information in the closedloop mode. Closed loop is theoperating mode where all enginecontrol sensors including the

Oxygen sensor are used to get bestfuel economy, lowest emissions,and good power.Should the O2 sensor be replacedwhen the sensor light comes on inyour car?Probably not, but you should testit to make sure it is alive and well.

This assumes that the light yousee is simply an emissions servicereminder light and not a failurelight. A reminder light is triggeredby a mileage event (20-40,000miles usually) or something like


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42000 key start cycles. EGR dashlights usually fall into thereminder category. Consult yourowners manual, auto repair

manual, dealer, or repair shop forhelp on what your light means.How do I know if my O2 sensor maybe bad?If your car has lost several milesper gallon of fuel economy and theusual tune up steps do notimprove it. This *is not* a pointerto O2 failure, it just brings up thepossibility. Vacuum leaks andignition problems are common fueleconomy destroyers. As mentionedby others, the on board computermay also set one of several failure"codes". If the computer has issued

a code pertaining to the O2 sensor,the sensor and it'swiring should betested. Usually when the sensor isbad, the engine will show someloss of power, and will not seem torespond quickly.What will damage my O2 sensor?Home or professional auto repairs

that have used silicone gasketsealer that is not specificallylabeled "Oxygen sensor safe","Sensor safe", or something similar,if used in an area that isconnected to the crankcase. This

includes valve covers, oil pan, ornearly any other gasket or sealthat controls engine oil. Leadedfuel will ruin the O2 sensor in a

short time. If a car is running richover a long period, the sensor maybecome plugged up or evendestroyed. Just shorting out thesensor output wire will not usuallyhurt the sensor. This simplygrounds the output voltage tozero. Once the wiring is repaired,the circuit operates normally.Undercoating, antifreeze or oil onthe *outside* surface of the sensorcan kill it. See how does an Oxygensensor 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, butmeasuring it's output voltage isnot harmful. As noted by otherposters, a cheap voltmeter will notbe accurate, but will cause nodamage. This is *not* true if youtry to measure the resistance of

the sensor. Resistancemeasurements send voltage into acircuit and check the amountreturning.


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5How does an O2 sensor work?An Oxygen sensor is a chemicalgenerator. It is constantly makinga comparison between the Oxygen

inside the exhaust manifold andair outside the engine. If thiscomparison shows little or noOxygen in the exhaust manifold, avoltage is generated. The output ofthe sensor is usually between 0and 1.1 volts. All spark combustionengines need the proper air fuelratio to operate correctly. Forgasoline this is 14.7 parts of air toone part of fuel. When the enginehas more fuel than needed, allavailable Oxygen is consumed inthe cylinder and gasses leavingthrough the exhaust contain

almost no Oxygen. This sends out avoltage greater than 0.45 volts. Ifthe engine is running lean, all fuelis burned, and the extra Oxygenleaves the cylinder and flows intothe exhaust. In this case, thesensor voltage goes lower than0.45 volts. Usually the output

range seen seen is 0.2 to 0.7 volts.The sensor does not begin togenerate it's full output until itreaches about 600 degrees F. Priorto this time the sensor is notconductive. It is as if the circuit

between the sensor and computeris not complete. The mid point isabout 0.45 volts. This is neitherrich nor lean. A fully warm O2

sensor *will not spend any time at0.45 volts*. In many cars, thecomputer sends out a bias voltageof 0.45 through the O2 sensor wire.If the sensor is not warm, or if thecircuit is not complete, thecomputer picks up a steady 0.45volts. Since the computer knowsthis is an "illegal" value, it judgesthe sensor to not be ready. Itremains in open loop operation,and uses all sensors except the O2to determine fuel delivery. Anytime an engine is operated in openloop, it runs somewhat rich and

makes more exhaust emissions.This translates into lost power,poor fuel economy and airpollution. The O2 sensor isconstantly in a state of transitionbetween high and low voltage.Manfucturers call this crossing ofthe 0.45 volt mark O2 cross counts.

The higher the number of O2 crosscounts, the better the sensor andother parts of the computercontrol system are working. It isimportant to remember that the O2sensor is comparing the amount of


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6Oxygen inside and outside theengine. If the outside of the sensorshould become blocked, or coatedwith oil, sound insulation,

undercoating or antifreeze,(among other things), thiscomparison is not possible.How can I test my O2 sensor?They can be tested both in the carand out. If you have a highimpedence volt meter, theprocedure is fairly simple. It willhelp you to have some backgroundon the way the sensor does it's job.Read how does an O2 sensor workfirst.Testing O2 sensors that areinstalledThe engine must first be fully

warm. If you have a defectivethermostat, this test may not bepossible due to a minimumtemperature required for closedloop operation. Attach the positivelead of a high impedence DCvoltmeter to the Oxygen sensoroutput wire. This wire should

remain attached to the computer.You will have to back probe theconnection or use a jumper wire toget access. The negative leadshould be attached to a goodclean ground on the engine block

or accessory bracket. Cheapvoltmeters will not give accurateresults because they load downthe circuit and absorb the voltage

that they are attempting tomeasure. A acceptable value is1,000,000 ohms/volt or more onthe DC voltage. Most (if not all)digital voltmeters meet this need.Few (if any) non-powered analog(needle style) voltmeters do. Checkthe specs for your meter to findout. Set your meter to look for 1volt DC. Many late model cars usea heated O2 sensor. These haveeither two or three wires insteadof one. Heated sensors will have 12volts on one lead, ground on theother, and the sensor signal on the

third. If you have two or threewires, use a 15 or higher volt scaleon the meter until you know whichis the sensor output wire. Whenyou turn the key on, do not startthe engine. You should see achange in voltage on the meter inmost late model cars. If not, check

your connections. Next, check yourleads to make sure you won't wrapup any wires in the belts, etc. thenstart the engine. You should runthe engine above 2000 rpm fortwo minutes to warm the O2 sensor


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7and try to get into closed loop.Closed loop operation is indicatedby the sensor showing severalcross counts per second. It may

help to rev the engine betweenidle and about 3000 rpm severaltimes. The computer recognizesthe sensor as hot and active oncethere are several cross counts. Youare looking for voltage to go aboveand below 0.45 volts. If you seeless than 0.2 and more than 0.7volts and the value changesrapidly, you are through, yoursensor is good. If not, is it steadyhigh (> 0.45) near 0.45 or steadylow (< 0.45). If the voltage is nearthe middle, you may not be hotyet. Run the engine above 2000

rpm again. If the reading is steadylow, add richness by partiallyclosing the choke or adding somepropane through the air intake. Bevery careful if you work with anyextra gasoline, you can easily beburned or have an explosion. If thevoltage now rises above 0.7 to 0.9,

and you can change it at will bychanging the extra fuel, the O2sensor is usually good. If thevoltage is steady high, create avacuum leak. Try pulling the PCVvalve out of it's hose and letting

air enter. You can also use thepower brake vacuum supply hose.If this drives the voltage to 0.2 to0.3 or less and you can control it

at will by opening and closing thevacuum leak, the sensor is usuallygood. If you are not able to make achange either way, stop theengine, unhook the sensor wirefrom the computer harness, andreattach your voltmeter to thesensor output wire. Repeat the richand lean steps. If you can't get thesensor voltage to change, and youhave a good sensor and groundconnection, try heating it oncemore. Repeat the rich and leansteps. If still no voltage or fixedvoltage, you have a bad sensor. If

you are not getting a voltage andthe car has been running richlately, the sensor may be carbonfouled. It is sometimes possible toclean a sensor in the car. Do thisby unplugging the sensor harness,warming up the engine, andcreating a lean condition at about

2000 rpm for 1 or 2 minutes. Createa big enough vacuum leak so thatthe engine begins to slow down.The extra heat will clean it off ifpossible. If not, it was deadanyway, no loss. In either case, fix


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8the cause of the rich mixture andretest. If you don't, the new sensorwill fail.Testing O2 sensors on the

workbench.Use a high impedence DCvoltmeter as above. Clamp thesensor in a vice, or use a plier orvice-grip to hold it. Clamp yournegative voltmeter lead to thecase, and the positive to theoutput wire. Use a propane torchset to high and the inner blueflame tip to heat the fluted orperforated area of the sensor. Youshould see a DC voltage of at least0.6 within 20 seconds. If not, mostlikely cause is open circuitinternally or lead fouling. If OK so

far, remove from flame. You shouldsee a drop to under 0.1 volt within4 seconds. If not likely siliconefouled. If still OK, heat for two fullminutes and watch for drops involtage. Sometimes, the internalconnections will open up underheat. This is the same a loose wire

and is a failure. If the sensor is OKat this point, and will switch fromhigh to low quickly as you movethe flame, the sensor is good. Bearin mind that good or bad isrelative, with port fuel injection

needing faster information thancarbureted systems. ANY O2 sensorthat will generate 0.9 volts ormore when heated, show 0.1 volts

or less within one second of flameremoval, AND pass the two minuteheat test is good regardless ofage. When replacing a sensor,don't miss the opportunity to usethe test above on the replacement.This will calibrate your evaluationskills and save you money in thefuture. There is almost always *no*benefit in replacing an oxygensensor that will pass the test inthe first line of this paragraph.

Rick Kirchhof Austin, Texas| Experience is what you

Domain: [email protected]| get when you don'tBang path:...!cs.utexas.edu!peyote!posms!rick| get what you want.Oxygen Sensors: What Do They Do?

Oxygen sensors are devices which

measure the amount of oxygen inthe gas or liquid that is beinganalyzed.

Today, oxygen sensors are beingused mainly in cars. They sense


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9the amount of oxygen in theexhaust gas being emitted fromthe car and determine if the air tofuel ratio is at an optimum. The

ideal or optimum air-fuel ratio issaid to be 14.7:1.

When the amount of air in theexhaust is greater, then themixture is said to be a leanmixture. In this case, the exhaustfumes will be full of nitrogenoxides. Such a mixture may oftencause an increase in the fuelefficiency of the car, but it maylead to an eventual damage to theengine.

When the amount of fuel in the

exhaust is greater, then themixture is said to be a richmixture. In this case, the exhaustfumes will be full of unburnt fuelvapors that are full ofhydrocarbons. This may cause anincrease in the power of the car,and a decrease in the carns fuel

economy. However, this caneventually lead to damage of thecatalytic converter of the car.

The engine management computeror ECU checks the data supplied by

the oxygen sensor against look-uptables and adjusts the air-fuelratio in order to have completecombustion occurring in the

engine.

Oxygen Sensor Testing: Importanceof Oxygen Sensor Testing

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10to damage and failure of theoxygen sensor and hamper theperformance of the car.

Therefore it is extremely essentialto know if the oxygen sensor isperforming as it should be. Themethod to test this is known asOxygen Sensor Testing.

How To Determine If The OxygenSensor Is Working Properly?

One can determine if the oxygensensor is working properly or not ifthe typical symptoms show up.

One symptom is if the carns fueleconomy drops or if the car begins

to lose power.

However, as a disclaimer, it wouldbe right to state here that thesesymptoms do not necessarily pointtowards only a failure of theoxygen sensor. But it could be oneof the factors or causes. Therefore

it would be the best to check it outwhilst one is looking into thecauses.

Most cars also have a sensor lighton the dashboard of the car.

However, one should consult theownersnmanual before rushing offto change the sensor when thelight comes on. More often than

not, the light is triggered when thecar has run a certain number ofmiles or has been started a certainnumber of times.

Oxygen sensor testing is themethod used by most automobilemechanics to see if the oxygensensor is in working order. To dothis, the equipment needed is ahigh impedance DC voltmeter. Onecan say that this voltmeter is anOxygen Sensor Tester.

Attach the positive lead of the

voltmeter to the oxygen sensornsoutput wire while it is attached tothe carns ECU. One can use jumperwires to gain this access as well.The negative lead of the voltmeteris to be attached to the ground oron the engine block or anaccessory bracket.

Set the voltmeter to look for 1 voltDC. In cold, the engine shouldoutput between 0.1 volts to 0.2volts while when the engine hasbeen running for over 20 minutes,


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1it would be warm and the outputshould fluctuate between 0.1 voltsto 0.9 volts.

Some cars use either 2 or 3 wiresinstead of 1 wire. In these cars, theheated sensors will give an outputof 12 volts on one lead, ground onthe second lead and the sensorsignal on the third lead. In thesecars, when you turn the key to mONnbut do not start the engine, therewill be a change in the signal.

This is a simple method of testingif the oxygen sensor is workingproperly or not.

This way, one can ensure that the

oxygen sensor in onens car is inproper working order and if not,one can take the necessary stepsto change, replace or repair it.

Oxygen Sensor Testing

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