Engine JEEP GRAND CHEROKEE 2002 WJ / 2.G Workshop Manual

(M)Malfunction Indicator Lamp (MIL) illuminated during engine operation if this DTC was recorded
(depending if required by CARB and/or EPA). MIL is displayed as an engine icon on instrument panel.
(G)Generator lamp illuminated
Generic Scan
Tool P-CodeDRB Scan Tool Display Brief Description of DTC
P0712 (M) Trans Temp Sensor Voltage Too Low Voltage less than 1.55 volts (4-speed auto. trans. only).
P0713 Trans Temp Sensor Voltage Too
HighTransmission fluid temperature sensor input above
acceptable voltage. Was MIL code 37.
P0713 (M) Trans Temp Sensor Voltage Too
HighVoltage greater than 3.76 volts (4-speed auto. trans.
only).
P0720 (M) Low Output SPD Sensor RPM,
Above 15 MPHThe relationship between the Output Shaft Speed Sensor
and vehicle speed is not within acceptable limits.
P0720 (M) Low Output Spd Sensor RPM Above
15 mphOutput shaft speed is less than 60 rpm with vehicle speed
above 15 mph (4-speed auto. trans. only).
P0740 (M) Torq Con Clu, No RPM Drop at
LockupRelationship between engine and vehicle speeds
indicated failure of torque convertor clutch lock-up system
(TCC/PTU solenoid)
P0743 (M) Torque Converter Clutch Solenoid/
Trans Relay CircuitsAn open or shorted condition detected in the torque
converter clutch (part throttle unlock) solenoid control
circuit. Shift solenoid C electrical fault - Aisin transmission
P0743 (M) Torque Converter Clutch Solenoid/
Trans Relay CircuitsAn open or shorted condition detected in the torque
converter part throttle unlock solenoid control circuit (3 or
4-speed auto. trans. only).
P0748 (M) Governor Pressur Sol Control/Trans
Relay CircuitsAn open or shorted condition detected in the Governor
Pressure Solenoid circuit or Trans Relay Circuit in JTEC
RE transmissions.
P0748 (M) Governor Pressure Sol Control/Trans
Relay CircuitsAn open or shorted condition detected in the governor
pressure solenoid or relay circuits (4-speed auto. trans.
only).
P0751 (M) O/D Switch Pressed (Lo) More Than
5 MinutesOverdrive override switch input is in a prolonged
depressed state.
P0751 (M) O/D Switch Pressed (LO) More Than
5 MinOverdrive Off switch input too low for more than 5
minutes (4-speed auto. trans. only).
P0753 (M) Trans 3-4 Shift Sol/Trans Relay
CircuitsAn open or shorted condition detected in the overdrive
solenoid control circuit or Trans Relay Circuit in JTEC RE
transmissions. Was MIL code 45.
P0753 (M) Trans 3-4 Shift Sol/Trans Relay
CircuitsAn open or shorted condition detected in the transmission
2-4 shift solenoid circuit (4-speed auto. trans. only).
P0756 AW4 Shift Sol B (2-3) Functional
FailureShift solenoid B (2-3) functional fault - Aisin transmission
P0783 (M) 3-4 Shift Sol, No RPM Drop at
LockupThe overdrive solenoid is unable to engage the gear
change from 3rd gear to the overdrive gear.
P0801 Reverse Gear Lockout Circuit Open
or ShortAn open or shorted condition detected in the transmission
reverse gear lock-out solenoid control circuit.
P0830 Clutch Depressed Switch Circuit Problem detected in clutch switch circuit.
P0833 Clutch Released Switch Circuit Problem detected in clutch switch circuit.
P0836 4WD Mux Switch Circuit
P0837 4WD Mux Switch Performance
25 - 10 EMISSIONS CONTROLWJ
EMISSIONS CONTROL (Continued)

(M)Malfunction Indicator Lamp (MIL) illuminated during engine operation if this DTC was recorded
(depending if required by CARB and/or EPA). MIL is displayed as an engine icon on instrument panel.
(G)Generator lamp illuminated
Generic Scan
Tool P-CodeDRB Scan Tool Display Brief Description of DTC
P1110 Decrease Engine Performance Due
To High Intake Air TemperatureIntake manifold air temperature is above the engine
protection limit. Engine power will be derated.
P1180 Decreased Engine Performance Due
To High Injection Pump Fuel TempFuel temperature is above the engine protection limit.
Engine power will be derated.
P1192 Intake Air Temp Sensor Voltage Low
P1193 Intake Air Temp Sensor Voltage High
P1194 O2 Heater Performance
P1195 (M) 1/1 O2 Sensor Slow During Catalyst
MonitorA slow switching oxygen sensor has been detected in
bank 1/1 during catalyst monitor test. (Also see SCI DTC
$66) (was P0133)
P1196 (M) 2/1 O2 Sensor Slow During Catalyst
MonitorA slow switching oxygen sensor has been detected in
bank 2/1 during catalyst monitor test. (Also see SCI DTC
$7A) (was P0153)
P1197 1/2 O2 Sensor Slow During Catalyst
MonitorA slow switching oxygen sensor has been detected in
bank 1/2 during catalyst monitor test. (Also see SCI DTC
$68) (was P0139)
P1198 Radiator Temperature Sensor Volts
Too HighRadiator coolant temperature sensor input above the
maximum acceptable voltage.
P1199 Radiator Temperature Sensor Volts
Too LowRadiator coolant temperature sensor input below the
minimum acceptable voltage.
P1280 Fuel System Relay Circuit
P1281 Engine is Cold Too Long Engine coolant temperature remains below normal
operating temperatures during vehicle travel (Thermostat).
P1282 Fuel Pump/System Relay Control
CircuitAn open or shorted condition detected in the fuel pump
relay control circuit.
P1283 Idle Select Signal Invalid ECM or fuel injection pump module internal fault condition
detected.
P1284 (M) Fuel Injection Pump Battery Voltage
Out-Of-RangeFuel injection pump module internal fault condition
detected. Engine power will be derated.
P1285 (M) Fuel Injection Pump Controller
Always OnFuel injection pump module relay circuit failure detected.
Engine power will be derated.
P1286 Accelerator Position Sensor (APPS)
Supply Voltage Too HighHigh voltage detected at APPS.
P1287 Fuel Injection Pump Controller
Supply Voltage LowECM or fuel injection pump module internal fault condition
detected. Engine power will be derated.
P1288 Intake Manifold Short Runner
Solenoid CircuitAn open or shorted condition detected in the short runner
tuning valve circuit.
P1289 Manifold Tune Valve Solenoid Circuit An open or shorted condition detected in the manifold
tuning valve solenoid control circuit.
P1290 High Pressure Solenoid Relay Ckt. CNG Fuel System Pressure Too High±Compressed
natural gas system pressure above normal operating
range.
P1291 No Temp Rise Seen From Intake
HeatersEnergizing Heated Air Intake does not change intake air
temperature sensor an acceptable amount.
WJEMISSIONS CONTROL 25 - 11
EMISSIONS CONTROL (Continued)

(M)Malfunction Indicator Lamp (MIL) illuminated during engine operation if this DTC was recorded
(depending if required by CARB and/or EPA). MIL is displayed as an engine icon on instrument panel.
(G)Generator lamp illuminated
Generic Scan
Tool P-CodeDRB Scan Tool Display Brief Description of DTC
P1291 (M) No Temperature Rise Seen From
Intake Air HeatersProblem detected in intake manifold air heating system.
P1292 CNG Pressure Sensor Voltage Too
HighCompressed natural gas pressure sensor reading above
acceptable voltage.
P1293 CNG Pressure Sensor Voltage Too
LowCompressed natural gas pressure sensor reading below
acceptable voltage.
P1294 (M) Target Idle Not Reached Target RPM not achieved during drive idle condition.
Possible vacuum leak or IAC (AIS) lost steps.
P1295 (M) No 5 Volts to TP Sensor Loss of a 5 volt feed to the Throttle Position Sensor has
been detected.
P1295 (M) Accelerator Position Sensor (APPS)
Supply Voltage Too LowAPPS supply voltage input below the minimum
acceptable voltage.
P1296 No 5 Volts to MAP Sensor Loss of a 5 volt feed to the MAP Sensor has been
detected.
P1297 (M) No Change in MAP From Start To
RunNo difference is recognized between the MAP reading at
engine idle and the stored barometric pressure reading.
P1298 Lean Operation at Wide Open
ThrottleA prolonged lean condition is detected during Wide Open
Throttle
P1299 Vacuum Leak Found (IAC Fully
Seated)MAP Sensor signal does not correlate to Throttle Position
Sensor signal. Possible vacuum leak.
P1388 Auto Shutdown Relay Control Circuit An open or shorted condition detected in the ASD or CNG
shutoff relay control ckt.
P1388 Auto Shutdown Relay Control Circuit An open or shorted condition detected in the auto
shutdown relay circuit.
P1389 No ASD Relay Output Voltage At
PCMNo Z1 or Z2 voltage sensed when the auto shutdown
relay is energized.
P1389 (M) No ASD Relay Output Voltage at
PCMAn open condition detected In the ASD relay output
circuit.
P1390 Timing Belt Skipped 1 Tooth or More Relationship between Cam and Crank signals not correct
P1391 (M) Intermittent Loss of CMP or CKP Loss of the Cam Position Sensor or Crank Position
sensor has occurred. For PL 2.0L
P1398 (M) Mis-Fire Adaptive Numerator at Limit PCM is unable to learn the Crank Sensor's signal in
preparation for Misfire Diagnostics. Probable defective
Crank Sensor
P1399 Wait To Start Lamp Cicuit An open or shorted condition detected in the Wait to Start
Lamp circuit.
P1403 No 5V to EGR Sensor Loss of 5v feed to the EGR position sensor.
P01475 Aux 5 Volt Supply Voltage High Sensor supply voltage for ECM sensors is too high.
P1476 Too Little Secondary Air Insufficient flow of secondary air injection detected during
aspirator test (was P0411)
P1477 Too Much Secondary Air Excessive flow of secondary air injection detected during
aspirator test (was P0411).
25 - 12 EMISSIONS CONTROLWJ
EMISSIONS CONTROL (Continued)

(M)Malfunction Indicator Lamp (MIL) illuminated during engine operation if this DTC was recorded
(depending if required by CARB and/or EPA). MIL is displayed as an engine icon on instrument panel.
(G)Generator lamp illuminated
Generic Scan
Tool P-CodeDRB Scan Tool Display Brief Description of DTC
P1478 Battery Temp Sensor Volts Out of
LimitInternal temperature sensor input voltage out of an
acceptable range.
P1479 Transmission Fan Relay Circuit An open or shorted condition detected in the transmission
fan relay circuit.
P1480 PCV Solenoid Circuit An open or shorted condition detected in the PCV
solenoid circuit.
P1481 EATX RPM Pulse Perf EATX RPM pulse generator signal for misfire detection
does not correlate with expected value.
P1482 Catalyst Temperature Sensor Circuit
Shorted LowCatalyst temperature sensor circuit shorted low.
P1483 Catalyst Temperature Sensor Circuit
Shorted High.Catalyst temperature sensor circuit shorted high.
P1484 Catalytic Converter Overheat
DetectedA catalyst overheat condition has been detected by the
catalyst temperature sensor.
P1485 Air Injection Solenoid Circuit An open or shorted condition detected in the air assist
solenoid circuit.
P1486 Evap Leak Monitor Pinched Hose
FoundLDP has detected a pinched hose in the evaporative hose
system.
P1487 Hi Speed Rad Fan CTRL Relay
CircuitAn open or shorted condition detected in the control
circuit of the #2 high speed radiator fan control relay.
P1488 Auxiliary 5 Volt Supply Output Too
LowAuxiliary 5 volt sensor feed is sensed to be below an
acceptable limit.
P1488 5 Volt Supply Voltage Low Sensor supply voltage for ECM sensors is too low.
P1489 High Speed Fan CTRL Relay Circuit An open or shorted condition detected in the control
circuit of the high speed radiator fan control relay.
P1490 Low Speed Fan CTRL Relay Circuit An open or shorted condition detected in control circuit of
the low speed radiator fan control relay.
P1491 Rad Fan Control Relay Circuit An open or shorted condition detected in the radiator fan
control relay control circuit. This includes PWM solid state
relays.
P1492 Ambient/Batt Temp Sen Volts Too
HighExternal temperature sensor input above acceptable
voltage.
P1492 (M) Ambient/Batt Temp Sensor Volts Too
HighBattery temperature sensor input voltage above an
acceptable range.
P1493 (M) Ambient/Batt Temp Sen Volts Too
LowExternal temperature sensor input below acceptable
voltage.
P1493 (M) Ambient/Batt Temp Sen Volts Too
LowBattery temperature sensor input voltage below an
acceptable range.
P1494 (M) Leak Detection Pump Sw or
Mechanical FaultIncorrect input state detected for the Leak Detection
Pump (LDP) pressure switch.
P1495 Leak Detection Pump Solenoid
CircuitAn open or shorted condition detected in the Leak
Detection Pump (LDP) solenoid circuit.
P1496 5 Volt Supply, Output Too Low 5 volt sensor feed is sensed to be below an acceptable
limit. ( less than 4v for 4 sec )
WJEMISSIONS CONTROL 25 - 13
EMISSIONS CONTROL (Continued)

(M)Malfunction Indicator Lamp (MIL) illuminated during engine operation if this DTC was recorded
(depending if required by CARB and/or EPA). MIL is displayed as an engine icon on instrument panel.
(G)Generator lamp illuminated
Generic Scan
Tool P-CodeDRB Scan Tool Display Brief Description of DTC
P1498 High Speed Rad Fan Ground CTRL
Rly CircuitAn open or shorted condition detected in the control
circuit of the #3 high speed radiator fan control relay.
P1499 Hydraulic cooling fan solenoid circuit An open or shorted condition detected in the cooling fan
control solenoid circuit.
P1594 (G) Charging System Voltage Too High Battery voltage sense input above target charging voltage
during engine operation.
P1594 Charging System Voltage Too High Battery voltage sense input above target charging voltage
during engine operation.
P1595 Speed Control Solenoid Circuits An open or shorted condition detected in either of the
speed control vacuum or vent solenoid control circuits.
P1595 Speed Control Solenoid Circuits An open or shorted condition detected in the speed
control vacuum or vent solenoid circuits.
P1596 Speed Control Switch Always High Speed control switch input above maximum acceptable
voltage.
P1597 Speed Control Switch Always Low Speed control switch input below minimum acceptable
voltage.
P1597 Speed Control Switch Always Low Speed control switch input below the minimum acceptable
voltage.
P1598 A/C Pressure Sensor Volts Too High A/C pressure sensor input above maximum acceptable
voltage.
P1598 A/C Sensor Input Hi Problem detected in air conditioning electrical circuit.
P1599 A/C Pressure Sensor Volts Too Low A/C pressure sensor input below minimum acceptable
voltage.
P1599 A/C Sensor Input Lo Problem detected in air conditioning electrical circuit.
P1602 PCM not programmed
P1680 Clutch Released Switch Circuit Problem detected in clutch switch electrical circuit.
P1681 No I/P Cluster CCD/J1850
Messages ReceivedNo CCD/J1850 messages received from the cluster
control module.
P1682 (G) Charging System Voltage Too Low Battery voltage sense input below target charging voltage
during engine operation and no significant change in
voltage detected during active test of generator output
circuit.
P1682 Charging System Voltage Too Low Charging system output voltage low.
P1683 SPD CTRL PWR Relay; or S/C 12v
Driver CKTAn open or shorted condition detected in the speed
control servo power control circuit.
P1683 Spd ctrl pwr rly, or s/c 12v driver
circuitAn open or shorted condition detected in the speed
control servo power control circuit.
P1684 Batt Loss (disconnected) in last 50
StartsThe battery has been disconnected within the last 50
starts
P1685 SKIM Invalid Key - (Wrong or Invalid
Key MSG Received From SKIM)The engine controler has received an invalid key from the
SKIM.
P1686 No SKIM BUS Messages Received No CCD/J1850 messages received from the Smart Key
Immobilizer Module (SKIM).
25 - 14 EMISSIONS CONTROLWJ
EMISSIONS CONTROL (Continued)

(M)Malfunction Indicator Lamp (MIL) illuminated during engine operation if this DTC was recorded
(depending if required by CARB and/or EPA). MIL is displayed as an engine icon on instrument panel.
(G)Generator lamp illuminated
Generic Scan
Tool P-CodeDRB Scan Tool Display Brief Description of DTC
P1687 No MIC BUS Message (No Cluster
BUS Message)No CCD/J1850 messages received from the Mechanical
Instrument Cluster (MIC) module.
P1688 (M) Internal Fuel Injection Pump
Controller FailureInternal problem within the fuel injection pump. Low
power, engine derated, or engine stops.
P1689 (M) No Communication Between ECM
and Injection Pump ModuleData link circuit failure between ECM and fuel injection
pump. Low power, engine derated, or engine stops.
P1690 (M) Fuel Injection Pump CKP Sensor
Does Not Agree With ECM CKP
SensorProblem in fuel sync signal. Possible injection pump
timing problem. Low power, engine derated, or engine
stops.
P1691 Fuel Injection Pump Controller
Calibration ErrorInternal fuel injection pump failure. Low power, engine
derated, or engine stops.
P1692 DTC Set In ECM A9Companion DTC9was set in both the ECM and PCM.
P1693 (M) DTC Detected in Companion Module A fault has been generated in the companion engine
control module.
P1693 (M) DTC Detected in PCM/ECM or DTC
Detected in ECMA9Companion DTC9was set in both the ECM and PCM.
P1694 Fault In Companion Module No CCD/J1850 messages received from the powertrain
control module-Aisin transmission
P1694 (M) No BUS (CCD) Messages received
from ECMBus communication failure to PCM.
P1695 No CCD/J1850 Message From Body
Control ModuleNo CCD/J1850 messages received from the body control
module.
P1696 PCM Failure EEPROM Write Denied Unsuccessful attempt to write to an EEPROM location by
the control module.
P1697 PCM Failure SRI Mile Not Stored Unsuccessful attempt to update Service Reminder
Indicator (SRI or EMR) mileage in the control module
EEPROM.
P1698 No CCD/J1850 Message From TCM No CCD/J1850 messages received from the electronic
transmission control module (EATX) or the Aisin
transmission controller.
P1698 No CCD Messages received from
PCMBus communication failure to PCM. A9Companion DTC9
was set in both the ECM and PCM.
P1699 No Climate Control Bus Messages
P1719 Skip Shift Solenoid Circuit An open or shorted condition detected in the transmission
2-3 gear lock-out solenoid control circuit.
P1740 TCC or OD Sol Perf A rationality error has been detected in either the TCC
solenoid or overdrive solenoid systems.
P1740 (M) TCC OR O/D Solenoid Performance Problem detected in transmission convertor clutch and/or
overdrive circuits (diesel engine with 4-speed auto. trans.
only).
WJEMISSIONS CONTROL 25 - 15
EMISSIONS CONTROL (Continued)

(M)Malfunction Indicator Lamp (MIL) illuminated during engine operation if this DTC was recorded
(depending if required by CARB and/or EPA). MIL is displayed as an engine icon on instrument panel.
(G)Generator lamp illuminated
Generic Scan
Tool P-CodeDRB Scan Tool Display Brief Description of DTC
P1756 (M) GOV Press Not Equal to Target @
15-20 PSIThe requested pressure and the actual pressure are not
within a tolerance band for the Governor Control System
which is used to regulate governor pressure to control
shifts for 1st, 2nd, and 3rd gear. (Mid Pressure
Malfunction)
P1756 (M) Governor Pressure Not Equal to
Target @ 15-20 PSIGovernor sensor input not between 10 and 25 psi when
requested (4-speed auto. trans. only).
P1757 GOV Press Not Equal to Target @
15-20 PSIThe requested pressure and the actual pressure are not
within a tolerance band for the Governor Control System
which is used to regulate governor pressure to control
shifts for 1st, 2nd, and 3rd gear (Zero Pressure
Malfunction)
P1757 (M) Governor Pressure Above 3 PSI In
Gear With 0 MPHGovernor pressure greater than 3 psi when requested to
be 0 psi (4-speed auto. trans. only).
P1762 (M) Gov Press Sen Offset Volts Too Lo
or HighThe Governor Pressure Sensor input is greater than a
calibration limit or is less than a calibration limit for 3
consecutive park/neutral calibrations.
P1762 (M) Governor Press Sen Offset Volts Too
Low or HighSensor input greater or less than calibration for 3
consecutive Neutral/Park occurrences (4-speed auto.
trans. only).
P1763 Governor Pressure Sensor Volts Too
HiThe Governor Pressure Sensor input is above an
acceptable voltage level.
P1763 (M) Governor Pressure Sensor Volts Too
HIVoltage greater than 4.89 volts (4-speed auto. trans.
only).
P1764 (M) Governor Pressure Sensor Volts Too
LowThe Governor Pressure Sensor input is below an
acceptable voltage level.
P1764 (M) Governor Pressure Sensor Volts Too
LowVoltage less than .10 volts (4-speed auto. trans. only).
P1765 (M) Trans 12 Volt Supply Relay CTRL
CircuitAn open or shorted condition is detected in the
Transmission Relay control circuit. This relay supplies
power to the TCC
P1765 (M) Trans 12 Volt Supply Relay Ctrl
CircuitCurrent state of solenoid output port is different than
expected (4-speed auto. trans. only).
P1830 Clutch Override Relay Circuit Problem detected in clutch pedal switch override relay
circuit.
P1899 (M) P/N Switch Stuck in Park or in Gear Incorrect input state detected for the Park/Neutral switch.
P1899 (M) P/N Switch Stuck in Park or in Gear Incorrect input state detected for the Park/Neutral switch
(3 or 4-speed auto. trans. only).
25 - 16 EMISSIONS CONTROLWJ
EMISSIONS CONTROL (Continued)

DESCRIPTION - TASK MANAGER
The PCM is responsible for efficiently coordinating
the operation of all the emissions-related compo-
nents. The PCM is also responsible for determining if
the diagnostic systems are operating properly. The
software designed to carry out these responsibilities
is referred to as the 'Task Manager'.
DESCRIPTION - MONITORED SYSTEMS
There are new electronic circuit monitors that
check fuel, emission, engine and ignition perfor-
mance. These monitors use information from various
sensor circuits to indicate the overall operation of the
fuel, engine, ignition and emission systems and thus
the emissions performance of the vehicle.
The fuel, engine, ignition and emission systems
monitors do not indicate a specific component prob-
lem. They do indicate that there is an implied prob-
lem within one of the systems and that a specific
problem must be diagnosed.
If any of these monitors detect a problem affecting
vehicle emissions, the Malfunction Indicator Lamp
(MIL) will be illuminated. These monitors generate
Diagnostic Trouble Codes that can be displayed with
the MIL or a scan tool.
The following is a list of the system monitors:
²Misfire Monitor
²Fuel System Monitor
²Oxygen Sensor Monitor
²Oxygen Sensor Heater Monitor
²Catalyst Monitor
²Leak Detection Pump Monitor (if equipped)
All these system monitors require two consecutive
trips with the malfunction present to set a fault.
Refer to the appropriate Powertrain Diagnos-
tics Procedures manual for diagnostic proce-
dures.
The following is an operation and description of
each system monitor:
OXYGEN SENSOR (O2S) MONITOR
Effective control of exhaust emissions is achieved
by an oxygen feedback system. The most important
element of the feedback system is the O2S. The O2S
is located in the exhaust path. Once it reaches oper-
ating temperature 300É to 350ÉC (572É to 662ÉF), the
sensor generates a voltage that is inversely propor-
tional to the amount of oxygen in the exhaust. The
information obtained by the sensor is used to calcu-
late the fuel injector pulse width. This maintains a
14.7 to 1 Air Fuel (A/F) ratio. At this mixture ratio,
the catalyst works best to remove hydrocarbons (HC),
carbon monoxide (CO) and nitrogen oxide (NOx) from
the exhaust.
The O2S is also the main sensing element for the
Catalyst and Fuel Monitors.The O2S can fail in any or all of the following
manners:
²slow response rate
²reduced output voltage
²dynamic shift
²shorted or open circuits
Response rate is the time required for the sensor to
switch from lean to rich once it is exposed to a richer
than optimum A/F mixture or vice versa. As the sen-
sor starts malfunctioning, it could take longer to
detect the changes in the oxygen content of the
exhaust gas.
The output voltage of the O2S ranges from 0 to 1
volt. A good sensor can easily generate any output
voltage in this range as it is exposed to different con-
centrations of oxygen. To detect a shift in the A/F
mixture (lean or rich), the output voltage has to
change beyond a threshold value. A malfunctioning
sensor could have difficulty changing beyond the
threshold value.
OXYGEN SENSOR HEATER MONITOR
If there is an oxygen sensor (O2S) shorted to volt-
age DTC, as well as a O2S heater DTC, the O2S
fault MUST be repaired first. Before checking the
O2S fault, verify that the heater circuit is operating
correctly.
Effective control of exhaust emissions is achieved
by an oxygen feedback system. The most important
element of the feedback system is the O2S. The O2S
is located in the exhaust path. Once it reaches oper-
ating temperature 300É to 350ÉC (572 É to 662ÉF), the
sensor generates a voltage that is inversely propor-
tional to the amount of oxygen in the exhaust. The
information obtained by the sensor is used to calcu-
late the fuel injector pulse width. This maintains a
14.7 to 1 Air Fuel (A/F) ratio. At this mixture ratio,
the catalyst works best to remove hydrocarbons (HC),
carbon monoxide (CO) and nitrogen oxide (NOx) from
the exhaust.
The voltage readings taken from the O2S sensor
are very temperature sensitive. The readings are not
accurate below 300ÉC. Heating of the O2S sensor is
done to allow the engine controller to shift to closed
loop control as soon as possible. The heating element
used to heat the O2S sensor must be tested to ensure
that it is heating the sensor properly.
The O2S sensor circuit is monitored for a drop in
voltage. The sensor output is used to test the heater
by isolating the effect of the heater element on the
O2S sensor output voltage from the other effects.
LEAK DETECTION PUMP MONITOR (IF EQUIPPED)
The leak detection assembly incorporates two pri-
mary functions: it must detect a leak in the evapora-
WJEMISSIONS CONTROL 25 - 17
EMISSIONS CONTROL (Continued)

tive system and seal the evaporative system so the
leak detection test can be run.
The primary components within the assembly are:
A three port solenoid that activates both of the func-
tions listed above; a pump which contains a switch,
two check valves and a spring/diaphragm, a canister
vent valve (CVV) seal which contains a spring loaded
vent seal valve.
Immediately after a cold start, between predeter-
mined temperature thresholds limits, the three port
solenoid is briefly energized. This initializes the
pump by drawing air into the pump cavity and also
closes the vent seal. During non test conditions the
vent seal is held open by the pump diaphragm
assembly which pushes it open at the full travel posi-
tion. The vent seal will remain closed while the
pump is cycling due to the reed switch triggering of
the three port solenoid that prevents the diaphragm
assembly from reaching full travel. After the brief
initialization period, the solenoid is de-energized
allowing atmospheric pressure to enter the pump
cavity, thus permitting the spring to drive the dia-
phragm which forces air out of the pump cavity and
into the vent system. When the solenoid is energized
and de energized, the cycle is repeated creating flow
in typical diaphragm pump fashion. The pump is con-
trolled in 2 modes:
Pump Mode:The pump is cycled at a fixed rate to
achieve a rapid pressure build in order to shorten the
overall test length.
Test Mode:The solenoid is energized with a fixed
duration pulse. Subsequent fixed pulses occur when
the diaphragm reaches the Switch closure point.
The spring in the pump is set so that the system
will achieve an equalized pressure of about 7.5º
water. The cycle rate of pump strokes is quite rapid
as the system begins to pump up to this pressure. As
the pressure increases, the cycle rate starts to drop
off. If there is no leak in the system, the pump would
eventually stop pumping at the equalized pressure. If
there is a leak, it will continue to pump at a rate rep-
resentative of the flow characteristic of the size of the
leak. From this information we can determine if the
leak is larger than the required detection limit (cur-
rently set at .040º orifice by CARB). If a leak is
revealed during the leak test portion of the test, the
test is terminated at the end of the test mode and no
further system checks will be performed.
After passing the leak detection phase of the test,
system pressure is maintained by turning on the
LDP's solenoid until the purge system is activated.
Purge activation in effect creates a leak. The cycle
rate is again interrogated and when it increases due
to the flow through the purge system, the leak check
portion of the diagnostic is complete.The canister vent valve will unseal the system
after completion of the test sequence as the pump
diaphragm assembly moves to the full travel position.
Evaporative system functionality will be verified by
using the stricter evap purge flow monitor. At an
appropriate warm idle the LDP will be energized to
seal the canister vent. The purge flow will be clocked
up from some small value in an attempt to see a
shift in the 02 control system. If fuel vapor, indicated
by a shift in the 02 control, is present the test is
passed. If not, it is assumed that the purge system is
not functioning in some respect. The LDP is again
turned off and the test is ended.
MISFIRE MONITOR
Excessive engine misfire results in increased cata-
lyst temperature and causes an increase in HC emis-
sions. Severe misfires could cause catalyst damage.
To prevent catalytic convertor damage, the PCM
monitors engine misfire.
The Powertrain Control Module (PCM) monitors
for misfire during most engine operating conditions
(positive torque) by looking at changes in the crank-
shaft speed. If a misfire occurs the speed of the
crankshaft will vary more than normal.
FUEL SYSTEM MONITOR
To comply with clean air regulations, vehicles are
equipped with catalytic converters. These converters
reduce the emission of hydrocarbons, oxides of nitro-
gen and carbon monoxide. The catalyst works best
when the Air Fuel (A/F) ratio is at or near the opti-
mum of 14.7 to 1.
The PCM is programmed to maintain the optimum
air/fuel ratio of 14.7 to 1. This is done by making
short term corrections in the fuel injector pulse width
based on the O2S sensor output. The programmed
memory acts as a self calibration tool that the engine
controller uses to compensate for variations in engine
specifications, sensor tolerances and engine fatigue
over the life span of the engine. By monitoring the
actual fuel-air ratio with the O2S sensor (short term)
and multiplying that with the program long-term
(adaptive) memory and comparing that to the limit,
it can be determined whether it will pass an emis-
sions test. If a malfunction occurs such that the PCM
cannot maintain the optimum A/F ratio, then the
MIL will be illuminated.
CATALYST MONITOR
To comply with clean air regulations, vehicles are
equipped with catalytic converters. These converters
reduce the emission of hydrocarbons, oxides of nitro-
gen and carbon monoxide.
Normal vehicle miles or engine misfire can cause a
catalyst to decay. This can increase vehicle emissions
25 - 18 EMISSIONS CONTROLWJ
EMISSIONS CONTROL (Continued)

and deteriorate engine performance, driveability and
fuel economy.
The catalyst monitor uses dual oxygen sensors
(O2S's) to monitor the efficiency of the converter. The
dual O2S's sensor strategy is based on the fact that
as a catalyst deteriorates, its oxygen storage capacity
and its efficiency are both reduced. By monitoring
the oxygen storage capacity of a catalyst, its effi-
ciency can be indirectly calculated. The upstream
O2S is used to detect the amount of oxygen in the
exhaust gas before the gas enters the catalytic con-
verter. The PCM calculates the A/F mixture from the
output of the O2S. A low voltage indicates high oxy-
gen content (lean mixture). A high voltage indicates a
low content of oxygen (rich mixture).
When the upstream O2S detects a lean condition,
there is an abundance of oxygen in the exhaust gas.
A functioning converter would store this oxygen so it
can use it for the oxidation of HC and CO. As the
converter absorbs the oxygen, there will be a lack of
oxygen downstream of the converter. The output of
the downstream O2S will indicate limited activity in
this condition.
As the converter loses the ability to store oxygen,
the condition can be detected from the behavior of
the downstream O2S. When the efficiency drops, no
chemical reaction takes place. This means the con-
centration of oxygen will be the same downstream as
upstream. The output voltage of the downstream
O2S copies the voltage of the upstream sensor. The
only difference is a time lag (seen by the PCM)
between the switching of the O2S's.
To monitor the system, the number of lean-to-rich
switches of upstream and downstream O2S's is
counted. The ratio of downstream switches to
upstream switches is used to determine whether the
catalyst is operating properly. An effective catalyst
will have fewer downstream switches than it has
upstream switches i.e., a ratio closer to zero. For a
totally ineffective catalyst, this ratio will be one-to-
one, indicating that no oxidation occurs in the device.
The system must be monitored so that when cata-
lyst efficiency deteriorates and exhaust emissions
increase to over the legal limit, the MIL will be illu-
minated.
DESCRIPTION - TRIP DEFINITION
The term ªTripº has different meanings depending
on what the circumstances are. If the MIL (Malfunc-
tion Indicator Lamp) is OFF, a Trip is defined as
when the Oxygen Sensor Monitor and the Catalyst
Monitor have been completed in the same drive cycle.
When any Emission DTC is set, the MIL on the
dash is turned ON. When the MIL is ON, it takes 3
good trips to turn the MIL OFF. In this case, itdepends on what type of DTC is set to know what a
ªTripº is.
For the Fuel Monitor or Mis-Fire Monitor (contin-
uous monitor), the vehicle must be operated in the
ªSimilar Condition Windowº for a specified amount of
time to be considered a Good Trip.
If a Non-Contiuous OBDII Monitor fails twice in a
row and turns ON the MIL, re-running that monitor
which previously failed, on the next start-up and
passing the monitor, is considered to be a Good Trip.
These will include the following:
²Oxygen Sensor
²Catalyst Monitor
²Purge Flow Monitor
²Leak Detection Pump Monitor (if equipped)
²EGR Monitor (if equipped)
²Oxygen Sensor Heater Monitor
If any other Emission DTC is set (not an OBDII
Monitor), a Good Trip is considered to be when the
Oxygen Sensor Monitor and Catalyst Monitor have
been completed; or 2 Minutes of engine run time if
the Oxygen Sensor Monitor or Catalyst Monitor have
been stopped from running.
It can take up to 2 Failures in a row to turn on the
MIL. After the MIL is ON, it takes 3 Good Trips to
turn the MIL OFF. After the MIL is OFF, the PCM
will self-erase the DTC after 40 Warm-up cycles. A
Warm-up cycle is counted when the ECT (Engine
Coolant Temperature Sensor) has crossed 160ÉF and
has risen by at least 40ÉF since the engine has been
started.
DESCRIPTION - COMPONENT MONITORS
There are several components that will affect vehi-
cle emissions if they malfunction. If one of these com-
ponents malfunctions the Malfunction Indicator
Lamp (MIL) will illuminate.
Some of the component monitors are checking for
proper operation of the part. Electrically operated
components now have input (rationality) and output
(functionality) checks. Previously, a component like
the Throttle Position sensor (TPS) was checked by
the PCM for an open or shorted circuit. If one of
these conditions occurred, a DTC was set. Now there
is a check to ensure that the component is working.
This is done by watching for a TPS indication of a
greater or lesser throttle opening than MAP and
engine rpm indicate. In the case of the TPS, if engine
vacuum is high and engine rpm is 1600 or greater
and the TPS indicates a large throttle opening, a
DTC will be set. The same applies to low vacuum if
the TPS indicates a small throttle opening.
All open/short circuit checks or any component that
has an associated limp in will set a fault after 1 trip
with the malfunction present. Components without
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