troubleshooting automotive computer systems pdf

TROUBLESHOOTING
AUTOMOTIVECOMPUTER
SYSTEMS



By








HappysonGavi

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CONTENTS





Chapter1.PCMinaBusNetwork
3


Chapter2.CANBUSSystem
5


Chapter3.OnboarddiagnosticsOBDI&OBDII18


Chapter4.AutomotiveComputerFundamentals
29


Chapter5.PCMtoSensorCommunication&StorageDevices33


Chapter6.PCMArchitecture
37


Chapter7.SensingDevices

44


Chapter8.IdealAir/FuelRatio
48


Chapter9.Sensorsusedtocalculateair/fuelratio56


Chapter10.IntroductiontoEmissionControl
72


Chapter11.AutomotiveEmissionControls
73



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Chapter12.DealingwithCommonDTCs
79


Chapter13.ChargingSystem
83

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Chapter1

1.1PCMinaBUSNetwork

The abbreviation PCM stands for Power Control Module. PCM is the main
automotive computer used to achieve various purposes mostly oriented toward
transmission, engine management and performance. The PCM is one of the
several on-board computers that are interconnected in an automotive vehicle
forming a network system called CAN BUS, (CAN) for Controller Area
Network, BUS is a form of network topology in which different in board
computers/modulesondifferentlocationsareconnectedtothemaincable.BUS
network topology is similar to passengers inside a bus sitting on seats that are
located on different locations but sharing the same passage. CAN network
provideanintercommunicationsystemofvariousvehiclecomputerstomakea
CAN BUS system. The CAN BUS system shares data to each computer
connectedtothedatalink.Eachcomputerinthedatalinkissometimescalleda
node.ThePCMwhichisthemaincomputerhastheultimatecontroloftheData
BUS.
InbriefthePCM’sprimaryfunctionistomanagePowertrain.Thisincludesthe
ignition system,fuelsystemandemissionsystem.PCM’sultimategoalisto
ensurethatthesensorsworkproperlyandthatthecarisnon-polluting.


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Fig1.1:AdiagramillustratingageneralBUSnetworktopology.

1.2WhatisBUStopology?

In such type of topology, a long backbone cable (twisted pair cable in CAN
BUS) is used to link all the devices in the network. Wires are twisted to
eliminateelectricalnoise.Twistedpaircableisusedtolinkallthedevicesinthe
network.Droplinesandtapsareusedtoconnectdifferentnodes/modulestothis
backbone.Thistopologyallowsonlyonedevicetotransmitatatimesinceall
nodes on the network share a common BUS. The node to transmit next is
determined by distributed access protocol. This network topology helps to
eliminateredundantwiringtothemodules/ECU ’ sthatneedthesamesensor
information.


1.2.1AdvantagesofBUStopology
Requireslesscablingandthereforelessexpensiveascomparedtomesh,starand
treetopologies.
Easiertoinstallthanothertopologies
Weightreductionduetofewercomponents


1.2.2DisadvantagesofBUStopology

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It ’ s difficult to add new devices It ’ s difficult to do reconfiguration It ’ s
difficulttoisolatefaultsinnodesLimitedcablelengthrequired
Limitednumberofnodesthatcanbeconnected

Thenumberofaccessoriesinsideautomobileincreasedrapidlyinthelastthree

decades. A typical late model vehicle will have 10 or more modules. This
therefore implies that the number of actuators, sensors, navigation,
entertainment, climate control etc. and their corresponding electronic control
modules increased as well. Since modern electronics is almost completely
digital this therefore implies for the need for efficient communication that is
immune to interruption and noise using defined protocol. The communication
betweenallon-boardcomputersrequiresanetwork.Thisnetworkallowsallonboard computers to communicate with each other as well as scan tool
communications.EngineersnotedthatBUSnetworktopologywasaneffective
modeofcommunicationandcameupwithCANBUSsystem

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Chapter2


2.1CANBUSsystem
A serial BUS that was developed in 1983 by Robert Bosch GmbH and was
officially released in 1986 at the Society of Automobile Engineers (SAE)
congressinDetroitMichiganforin-vehiclenetworkincars.CANBUSemploy
twisted wires to eliminate radio frequency interference (RFI) and
electromagneticinterference(EMI)fromenteringthesystem.Thetwistedwire
isterminatedateachendwith120OhmresistorcalledaTerminationResistor.
Thetotalresistanceofthetwoterminatingresistorsconnectedinatwistedpair
BUSsumsupto60Ohmssincethe2-resistorsareconnectedinparallel.Ifone
endofatwistedpairwireisopena120Ohmwillbemeasuredacrosspin6and
14 of the data link connector (DLC) with the ignition switch turned off and
negativebatteryterminalbeingdisconnected.IfbothwiresareopenanOLwill
beindicatedonaDMM.SuchCANBUSisnotreliablefordatatransmission
andinmanycasesitmightfailtooperate.

CANisthefastestnetworkanditstwistedwirecantransmitatspeedsuptoone
millionbitspersecond.
CANnetworkallowdifferentmodulestosharecommonsensordatalikevehicle
speed, outside air temperature, coolant temperature and density of air. Such
information is essential for fuel trim and transmission shifting. Examples of
modulesthatwereprogrammedtousesamevehiclespeedsignalarepowertrain
control module (PCM), cruise control module (CCM), anti-lock brake control
module (ABS)and driver ’ s door module (DDM) CAN system was also
designed to function in the harsh automotive / truck environment. The CAN
BUS is one of the 5-protocols used in the on-board diagnostics or OBD2
diagnosticstandard.TheOBD2hasbeenmaintainedforallcarsandlighttruck

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soldintheUSAsince1986.






2.1.1Why120Ohmsterminatingresistor?
The120Ω cameupasaresultofconsideringtheTransmissionLineTheoryof
PhysicsThistheorytakesintoconsiderationthelengthofBUSwires
Thelengthisdeterminedintermsofwavelengths
The terminating resistors prevent signal reflections causing interference All
devicesinthenetworkhavetoconformtotheBUSimpedanceWhenCANBUS
isat60Ω (meaningtwo120Ω inparallelasshowninFig2.1.1)theBUScan
absorb all energy for maximum efficiency of the system

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Fig 2.1.1: illustration of a CAN BUS network with 120 ohm terminating
resistors



2.2CANCommunicationsProtocols
Protocolsareasystemofdigitalrulesorproceduresfordataexchangewithinor
between computers. More precisely Protocols have more to do with the way
dataisformatted,transmittedandreceived.
ISO9131-2
ISO13230-4(KeywordProtocol2000)
ISO15765-3SAEJ2480
J1859PWM(ClassB)
J1859VPW


2.2.1ClassesofCANNetwork
ClassA =onewirelowspeeddata,lessthan10Kbs,generallyusedfortrip
computersandentertainment.
ClassB=twowiremidspeeddata,10-125Kbs,generallyusedforinformation
transferamongmodulessuchastemperaturesensordata.
ClassC=twisted2-wirehighspeedforPCM,ECM,Airbags,Antilockbrakes.

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Classisbasically100timesfasterthanClassB.Class6&14oftheDLC

ClassD = is at speeds of up to 1.0 Mbit / second and appears on some late
modelcars.


When CAN communication is possible or communicates with the scan tool it
implies no wiring problems. The technician’s job dealing with CAN BUS
networkistoensurethat:
The CAN BUS network is in good condition. The battery has proper B+

voltage12.6V-13.5V
Nolooseconnectionexistsonawireharness
Allthecontrolunitsareoperatingproperly
GoodgroundonB-(GND)pin4&5ofDLC
ProperB+voltageonpin16ofDLC
IfalltheaboveconditionsaremetthentheCANBUSsystemcanbereliedon
suchthattroubleshootingandrepairwon’tbeabigdeal.Iwillexplainthatin
the proceeding chapters. Problems that normally occur in the BUS
communication are opens, shorts, and unwanted resistance. Diagnosing and
troubleshootingtheseproblemsrequiresaDMMandananaloguemeter.Iwill
explain the diagnosis and troubleshooting procedures in the accompanying
chapters.


2.2.2SummaryofCANfeatures
FasterthanotherBUScommunicationprotocols,withmaximumspeedof1000
000bps.
Costeffectivebecauseiteliminatesredundantwiringanditisaneasiersystemto

useascomparedtoothersLessaffectedbyEMIandRFduetotwistedwires
Message based rather than address based which makes it easier to expand No
wakeupneededbecauseitisa2-wiresystem

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Supportsupto15modulesincludingascantool
Uses a120 Ω terminating resistor at the ends of each pair to reduce electrical
noiseApplies2.5Vonbothwires:
A CAN_H (High) signal exists from 2.5V to a high approximately 3.5V when
active A CAN_L (Low) signal exists from 2.5 V to a low approximately 1.5V
when active Possibility of assigning priority to messages and guaranteed
maximumlatencytimes[1]
Detectionofpossiblepermanentfailuresofnodesandautomaticswitchingoffof

defectivenodes[2]




2.3VoltagelevelsofCANBUSDigitalSignals
CANusesadifferentialtypeofmodulecommunicationwherethevoltageofone
wireisequalbutoppositetovoltageofanotherwire.Whennocommunication
occursbothwireshaveavoltageof2.5Vapplied.So2.5Visareferencelevel.
IfthereisnodatabeingsentthevoltageatCAN_H(High)andCAN_L(Low)
willbe2.5ViftheDMMisconnectedtothechassisGNDwhichhappenstobe
pin4.
ThevoltagedifferencebetweenthepeakofCAN_H(+)andthepeakofCAN_L
(-)≈2Voltsinamplitude.

PeakCAN_H(+)-PeakCAN_L(-)≈2Volts
Whencommunicationisoccurring,CAN_H(High)goesby1Voltupto3.5Volts
andCAN_L(Low)goesdown1Voltto1.5Volt.


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2.3.1CANElectronCurrent
Currentcarryingwirehasmagneticfieldaroundit,suchfieldsinducesvoltage
butinoppositedirectionaccordingtoLenz’sLawofelectromagnetism.This
inducedvoltagetermedvoltagespikecandestroyelectroniccomponentsthatare
fundamentaltoanautomotivesystemandthereforetwistedpaircableisagood
ideatocurbsuchproblems.CAN_HandCAN_Lwiresaretwistedpairand
thereforetheelectronsignalcurrentwillflowindifferentdirectionsandthat
signalisinastateofbalanced.ThismakestheCANBUSimmunetoelectrical
noisesinceanyelectricalnoiseandinducedvoltagepresentinonewireexistsin
theotherbutinoppositedirectionsandthereforecancellationeffectwilloccur.
Thiswillminimisesignallossandincreasesignalconductivityofthelowerlevel
oftheCANsignal.


2.3.3MeasuringtheResistanceofCANBUSnetwork
Theruleofthumbsaysmeasuretheresistancewhennocurrentisflowing.
MeasuringtheresistanceoftheCANnetworkisdoneonpin6&14ofthe
DLC.ExtremecautionshouldbeexercisedwheneverdealingwiththeDLC
sincethepinsonDLCareverydelicateandvulnerable.Thefollowingstepsto

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betakenwhenmeasuringtheresistanceoftheCANnetwork;
BeforemeasuringtheCANnetworkresistancedisconnectB-ofthe

batteryfirst.Thisisbecausesomevoltagewillstillbepresentonpin6&
14evenatKeyOff.TheB-isdisconnectedfirstbecauseelectriccurrent
flowsfromnegativetopositive.
Waitformaximumof10minutesbeforeprobingpin6&14withaDMM

toensurethatvoltageisnegligible.
Measuretheresistanceonpin6&14afteryouaresurethatvoltagedoes

notexistonthosepins
TheDMMtestleadsshouldbeonpin6&14,redtestleadonCAN_H

(High)andblacktestleadonCAN_L(Low)


2.3.4CANBUSnetworkresistancevaluesinterpretation
Ifbothwiresareopenan “ OL ”willbeindicatedonaDMM
Ifoneendofatwistedpairwireisopena120Ωwillbemeasuredacross

pin6and14ofthedatalinkconnector(DLC),thisshowsthatthetwo
terminatingresistorsarenolongerconnectedinparallel.
A0.0Ωorverylowresistancefarmuchlessthan60Ωimpliesthatthe

twistedwiresareshortedtogether
Thedatabuswillremainoperationalwhenoneofthetwomodules

containingaterminatingresistorisnotconnectedtothenetwork.
Howeverthedatabuswillfailwhenbothterminatingresistorsarenot
connectedtothenetwork.
Doathoroughvisualinspectionandlookfornetworkwireinsulationdamage,
repairorevenreplacethewireifpossible,lookforlooseconnectionsaswell.
Repairthedamagedresistorsifpossible.Areadingof60Ωimpliesaperfect
connectionbetweendifferentcontrolunitsandtheCANBUS.

2.3.5MeasuringVoltageofCANBUSnetwork
Theruleofthumbsaysmeasurevoltagewhencurrentisflowinginthecircuit.
ThisisdonetomonitorcommunicationsandtochecktheCANBUSforproper

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operation.Thefollowingstepsaretobetakenwhenmeasuringvoltageofthe
CANnetwork;
Checkthebatteryvoltage,thebatteryvoltageshouldbeatleast75%state

ofchargebeforeMeasuringVoltageofCANBUSnetwork
UseaDMMsettoDCvolts
Pin4/5,6&14tobeusedforvoltagemeasurements
0.0Voltsindicatesshort-to-ground,checkfortheshortbydisconnecting

onemoduleatatimeuntilamodulecausingproblemsisfound.
Whennocommunicationoccursbothwireshaveavoltageof2.5Vapplied

andthisphenomenoniscalledrecessivestate.
Signalthatis12Voltsallthetimeindicatesshort-to-voltage,theBUS

circuitcouldbeshortedto12Volts.Checktherepairhistoryofthecar
withthecustomerbeforedoinganything,andthenstartunpluggingone
moduleatatimeuntilaproblemisfound.
VariablevoltageindicatesanormaloperationoftheCANBUS;this

usuallyindicatesthatmessagesarebeingtransmitsent(Tx)andreceived
(Rx)byatransceivercircuitembeddedindifferentmodules.

BatteryVoltage

Stateofcharge

12.66

100%

12.45

75%

12.24

50%

12.06

25%

11.89

0%




Summaryofnetworkcommunicationdiagnosis
Checkthebatteryvoltage,thebatteryvoltageshouldbeatleast75%stateof
chargebeforedoinganyelectricaldiagnosis.

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Doathoroughvisualinspectiontocheckaccessoriesthatdonotfunction
properlyasthiswillhelptoidentifyamoduleoraBUScircuitatfault.Apower
windowthatdoesnotworkproperlycanbeduetoaBCMproblem.A
miscommunicationbetweenanengineandtransmissioncanbeattributedtoa
TCMinsomecars.IrememberfixingaNissanextera2001modelthathadsuch
problem,itwascrankingwithnostartcondition.
Useascantoolusuallyafactoryscantooltoperformmodulestatustest.Check
ifthecomponents/systems/actuatorscanbecontrolledbythescantoolCheck
CANBUSnetworkforvoltages
DisconnectthePCMterminalsandcheckforcorrosion,sprayanti-corrosion
solventtoremoverustonPCMpins,W40isagoodsolventtoachievethat.
CorrosioncausesPCMintermittentmalfunction.
Checkforlooseterminalsinaconnector
Checkforloosegroundconnections
Checkowner’smanualforproceduresandstepstofixBUSnetworkproblems

2.4Nonetworkcommunicationdiagnosis
Checkthebatteryvoltage,thebatteryvoltageshouldbeatleast75%state
ofchargebeforedoinganyelectricaldiagnosis
Checkpin16oftheDLCforthepresenceofB+voltage.Atleast10Volts

shouldbeavailableatpin16duringcrankingwhenthebatteryisgood.If
novoltageisfoundonpin16checkthe(1)B+cabletothePCMif
properlyconnected,ifproperlyconnectedcheckfor(2)adamagedPCM
fuseorcircuitbreaker,PCMfusesarenormallydamagedwhentheB+
polarityisreversed.TheB+pinwhichisfusedmaintainsthePCM’s
volatilememoriessuchasKAMandDTC’smemorieswhentheignitionis
turnedoff.If(1)&(2)isvalidcheckforanylooseconnectionsinthewire
harnessthatmightcauseanocommunication.Iftheabovementioned
stepsarevalidthefollowingprocedureslistedbelowmaybedone:

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Besurethatyourscantoolisnotaproblem;youmayaswelltestitona
differentcar.
Testfor5voltreferencesignalfromanyaccessiblesensorslikeMAF,

TPSandMAP.The5voltreferenceoriginatesfromthePCM.Ifthe5
voltreferencesignalispresentthenthePCMispartiallyoperational,and
ifno5voltreferenceexiststhenoneofthesensorsisshortedtoground,
implyingthatitpullsallthe5voltreferencetotheground.Thebest
proceduretodealwiththatproblemwillbetodisconnectonesensorafter
theotheruntilthe5voltreferenceappearsonline.Thiscouldbevery
monotonousbutwehavetodoit.

InsomecasescheckthePCMrelay
CheckthePCMforanyexternaldamages,PCMcanbedamagedby

overchargingalternator,alternatorwithshortedrotorexcessivemoistureor
anycatastrophicoccurrences
Useanoscilloscopetocheckforvoltagemeasurementsontestingpoints,

inthisexerciseyouhavetoopenthePCMbeforethetestingandthen
afterwardsrunthetestingwhilsttheignitionisrunning.
ConsultaPCMconnectordiagramtodeterminetheconfigurationofthe

connectorpins


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Fig2.4:AschematicdiagramofaMAPsensorofoneoftheGMcars.TheMAP
sensorhas3-wiresassignifiedbyA,B&C.WiresCwhichisgreyincolouris
the5-voltreferencesignalthatisusedfortestingdescribedabove.Theabove
diagramcanbepulledfromALLDATAonlineprogram.Forthosewithout
accesstotheinternetandsubscriptionstoALLDATAaT-pintogetherwitha
DMMcanbeusedtoidentifythe5-Voltreferencesignal.Thatgreywirecanbe
inspectedtocheckifnotshortedtotheground[3].



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2.5EffectsofbadPCMgrounds
BadPCMgroundscancauseseriousproblemstothecomputerduetoincorrect
voltagedrop.ThePCMwasdesignedtoworkwith5Vreferenceimplyingthat
anyvoltagedropwillaffectthenormaloperationofacomputer.Listedbelow
aresomeoftheproblemscausedbypoorPCMground:Nostartconditions
AbnormalPCMreferencevoltage
Wonderingtroublecodes
Intermittentdriveabilityproblems
Poorengineperformance


2.6OBD2DataLinkConnector(DLC)
DLC or Data Link Connector is an OBDII standardized and has got 16 pin
configurations. It has been used in all automobiles manufactured since 1996.
DLC design and location depends on the manufacturer however most vehicles
haveDLClocatedunderthedash.Somepinsfunctionisstandardlikepin(4,5,
6,14&16)andrequiredbyallmanufacturers,whileotherslike(1,8&12)are
left to the individual manufacturer’s discretion. DLC is very delicate and
vulnerableandextracareandcautionisrequiredwheneverdealingwithit.
DLChaspins1-8runningontheuppersegmentofDLCand9-16runningonthe
lower
segment
of
the
DLC
as
shown
below:

Fig2.6:AdiagramshowingstandardOBDIIDataLinkConnectorwith16pin
configurations


2.6.1TheDLCallowsyouto:
TurnON/OFFactuatorsusingabidirectionalscantool

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TestSignal&Chassisground
Testingofcomputernetworksystem,protocolcommunication&

signalling
ProvidespowertotheScantool
Inbriefitisadiagnostictool


1OEMCOMM
2BUS+veline(squarewavesignal)
3OEMreserved
4Chassisground
5Signal/Sensorground
6CAN_H
7KLineISO9141Protocol
8OEMreserved
9OEMCOMM
10BUS–ve
11,12&13OEMreserved

14CAN_L
15LLineISO9141Protocols
16B+voltage



Differentmoduleshavedifferentnetworkspeeds.Networkspeedsareclassified
intodifferentclassesi.e.ClassA,ClassB,ClassC&ClassD.Electricalpulses
whicharesquarewaveSignalsareusedtocommunicateonaSerialline.
Electricalpulsesareaseriesof0’sand1’s.Logic“1”isat5VandLogic“0”is
at0V.TTL(Transistor-Transistor-Logic)istheLogiccircuitrylanguageinside
variouscomputers,itsvoltagelevelsarerepresentedbya“1”or“0”,Logic‘1”
beingat5V&“0”beingat0V..EachoftheLogic“1”or“0”iscalledabit.A
byteismadeupof8-bits.Eachbyterepresentsasmallamountofinformationor

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data.AchainofbytesconstitutetoamessagewhenitissenttoaCANBUS
network.The“1’s”or“0’s”dictatesthetype,origin&importanceofthesignal.
Theframeofinformationisbrokendowntostart,identifier&prioritysections.
Theprioritysectionisindicatedby“1”or“0”.Asignalwith“0”hashigh
prioritythanasignalwith“1”.Thisisdonetopreventcollisionproblemswithin
anetworkandallowssignalsfrommodulesliketheABSControllertohavea
higherprioritythanmodulesforpowerseat,powerwindowandclimatecontrol.


2.6.2AccessingOBD2DataLinkConnector(DLC)pins
Breakoutbox(BOB)offerseasyaccesstoDLCpinsfordiagnostic
measurements.TheBOBisatruereflectionoftheDLC.TheBOBprotectsthe

possibledamageofDLCifprobedwithDMMtestleads.BOBhas16pinsthat
matchthe16pinsontheDLC.Pin2,4,5,6,7,10&16haveLEDsthatlightup.
LEDsareusedtoindicatestatusofpower,groundsandcommunicationactivity
oneachofthepinslistedaboveoftheDLC.
Pins2,6,7,10lightuptoidentifyvehicledataOBD-IIprotocolandactivity.
ACAN_H&CAN_Lvoltageappearsonpin6&14ofDLCrespectively.Pin
16hasB+voltageatKeyONandchargingvoltageatKeyONEngineON.The
intensityofbrightnessofLEDonpin16increasesduringchargingascompared
toKeyON.ThediagrambelowillustratestheBOBwiththe16pins
configurationsandtwoconnectors.OneconnectorwillbeconnectedtotheDLC
andtheothertothescantool.


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Fig2.6.2:illustrationofOBD2Breaker-out-box





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Chapter3

3.1OnboarddiagnosticsOBDI&OBDII
OBD1wasthegenerationofautomotiveenginecontrolsysteminstitutedbythe
government.OBDIwasmeanttokeeptrackoflimitedamountofinformation

makinguseofO2sensor,EGRsensorandfueltrimunlikeOBDIIwhichismore
complexandmakesuseofvarioussensorsOBD11isasecondgenerationof
automotiveenginecontrolsysteminstitutedbythegovernmentwithmuch
stricterstandardsanduniformity.TobemorepreciseOBDIIwasdesignedto
detectelectrical,chemicalandmechanicalfailuresinthevehicleemission
controlsystemthatmightthreatenemissionlevels.Thisbookisgoingtofocus
onlyonOBDIIautomobiles.
Idealemissionlevelsaftercombustioninavehicle:


O2=0.5%
CO2=12-15%
CO=0.5%

3.1.1OBDIIMonitors
OBDIIcheckstheentiresystemunlikeOBDI.Itrunsaseriesoftestscalled
monitors.OBDIImonitorschecksinformationfromvarioussensorsfor
rationalitysuchprocessiscalledarationalitycheck.

3.1.2WhatexactlyareOBDIIMONITORS?
Monitorsareexperiments,whenyouperformanexperimentinalabyouexpect
togetsomeresults,analysetheresultsanddrawsomeconclusions.Thisis
analogoustotheconceptofoperationofmonitors.Monitorshavetheabilityto
runtestsofmultiplecomponentsandmakejudgementsaboutsystemefficiency.

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Tobemoreprecisewecansafelysaythatmonitorsarecarefullyplanned
experimentsdonebythePCMtoverifyifthesensorsinasubsystemare

workingtogethertoproduceacceptableemissions.Belowisalistofexamples
ofmonitorsthatmayberunbytheOBDIIsystem:


ComprehensivecomponentmonitorsMisfiredetectionmonitorsFuelsystem
monitors
HeatedCatalystmonitors
CatalystEfficiencymonitorsEGRsystemmonitors
EvaporativesystemmonitorsHeatedOxygensensormonitorsAirinjection
monitors



3.1.3Whendomonitorsrun?
Thefollowing3-monitorsruncontinuously:
Comprehensivecomponentmonitor(CCM)
MisfireDetectionmonitor(MDM)
Fuelsystemmonitor(FSM)


CCM–Checkcomputercontrolleddevicesforopens,rationality&shorts-togrounds.
MDM–Preventstheconditionsthatthreatenstheefficiencyofthecatalytic
converterFSM-Preventstheconditionsthatthreatenstheefficiencyofthe
catalyticconverterTheremainingmonitorsrunoncepertrip.Thenextchapter
willexplainthemeaningofthetermtripasusedinOBDIIsystem.

Thesolepurposeofrunningmonitorscontinuouslyistoprotectacatalytic
converterfromimminentthreatssuchasoverfuellingandenginemisfire.These
twothreatscancompletelydestroythecatalyticconverterandthatwillposea

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seriousthreattoourenvironment.IregardOBDIIasoneofthegreatest
scientificinventiondoneintheUSAconsideringthethreatsposedbyfossilfuels
totheenvironment.Globalwarmingiscausingcatastrophicweatherchanges
acrosstheglobeandifwedon’ttakeseriousstepthewholeworldwillbevery
muchinhabitable.ThankstothescientistwhocameupwiththeideaofOBDII.


3.1.4Whendootherremainingmonitorsrun?
DependsontheenablingcriterialikeforexampleitiswisetotestanO2sensor
duringaclosedloopstatusotherwisefueltrimisnotaccurate.Insimilarfashion
itiswisetotestEGRflowwiththeenginerunningsincetheEGRisclosedat
idle.Mostofthetestshavetobedonewhenthecoolantisundernormal
operatingtemperaturesotherwisethetestswon’tberational.ThePCMhasto
waitfornormaloperatingconditionstobemetbeforerunningthemonitors.





Thefollowingmonitorsrunnon-continuous

HeatedCatalystmonitors
CatalystEfficiencymonitors
EGRsystemmonitors
Evaporativesystemmonitors
HeatedOxygensensormonitors
Airinjectionmonitors

3.1.5HowdoesthePCMknowwhenconditionsarerighttorun
thetests?

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