
When Monitored:
At a cold start, engine coolant below 98°F, Ambient/Battery Sensor readingwithin27°F,andEngineCoolant
Temperature above 170°F on the previous key off.
Set Condition:
The Oxygen Sensor signal voltage is below 0.156 of a volt for 28 seconds after starting engine.
Possible Causes
(K243) O2 SENSOR 2/2 SIGNAL CIRCUIT SHORTED TO GROUND
(K243) O2 SENSOR 2/2 SIGNAL CIRCUIT SHORTED TO (K900) SENSOR GROUND CIRCUIT
(K243) O2 SENSOR 2/2 SIGNAL SHORTED TO THE (Z1) HEATER GROUND CIRCUIT
O2 SENSOR
PCM
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE -
DIAGNOSIS AND TESTING).
Diagnostic Test
1.O2 SENSOR BELOW 0.16 OF A VOLT
Ignition on, engine not running.
With the scan tool, read DTCs and record the related Freeze Frame data.
Start the engine.
Allow the engine to idle for 4 to 5 minutes.
With the scan tool, read the O2 Sensor voltage.
Is the voltage below 0.16 of a volt?
Ye s>>
Go To 2
No>>
Refer to the INTERMITTENT CONDITION Diagnostic Procedure.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
2.O2 SENSOR OPERATION
WARNING: When the engine is operating, do not stand in a direct line with thefan. Do not put your hands
near the pulleys, belts, or fan. Do not wear loose clothing.
Disconnect the O2 Sensor harness connector.
Start the engine.
With the scan tool, monitor the O2 Sensor voltage.
IstheO2Sensorvoltageabove0.16ofavolt?
Ye s>>
Replace the O2 Sensor
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 3

When Monitored:
With battery voltage greater than 10.4 volts, engine running for more than4 minutes and coolant temperature
above 180°F.
Set Condition:
The oxygen sensor voltage is above 1.5 volts.
Possible Causes
(K243) O2 SENSOR 2/2 SIGNAL SHORTED TO VOLTAGE
(K243) O2 SENSOR 2/2 SIGNAL CIRCUIT OPEN
(K243) O2 SENSOR 2/2 SIGNAL CIRCUIT SHORTED TO THE (K344) O2 HEATER RELALY OUPUT CIRCUIT
(K900) SENSOR GROUND CIRCUIT OPEN
(Z1) O2 SENSOR HEATER GROUND CIRCUIT OPEN
O2 SENSOR
PCM
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE -
DIAGNOSIS AND TESTING).
Diagnostic Test
1.O2 SENSOR ABOVE 1.5 VOLTS
Ignition on, engine not running.
With the scan tool, read DTCs and record the related Freeze Frame data.
Start the engine.
Allow the engine to idle for 4 to 5 minutes.
With the scan tool, read the O2 Sensor voltage.
Is the voltage above 1.5 volts?
Ye s>>
Go To 2
No>>
Refer to the INTERMITTENT CONDITION Diagnostic Procedure.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
2.O2 SENSOR OPERATION
WARNING: When the engine is operatind, do not stand in a direct line with thefan. Do not put your hands
near the pulleys, belts, or fan. Do not wear loose clothing.
Turn the ignition off.
Disconnect the O2 Sensor harness connector.
Start the engine.
With the scan tool, monitor the O2 Sensor voltage.
Is the O2 Sensor voltage below 1.5 volts?
Ye s>>
Replace the O2 Sensor.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 3

2.CHECKING FUEL PRESSURE
WARNING: The fuel system is under a constant pressure (even with the engineoff). Before testing or ser-
vicing any fuel system hose, fitting or line, the fuel system pressure mustbe released.
Install a fuel pressure gauge.
Ignition on, engine not running.
With the scan tool, actuate the ASD FuelSystem test and observe the fuel pressure gauge.
NOTE: Fuel pressure specification is 379 KPa +/- 14 KPa (55 psi +/- 2 psi).
Turn the ignition off.
Choose a conclusion that best matches your fuel pressure reading.
Below Specification
Go To 3
Within Specification
Go To 6
Above Specification
Replace the fuel filter/pressure regulator.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
CAUTION: Stop All Actuations.
3.RESTRICTED FUEL SUPPLY LINE
Turn the ignition off.
WARNING: The fuel system is under a constant pressure (even with the engineoff). Before testing or ser-
vicing any fuel system hose, fitting or line, the fuel system pressure mustbe released.
Raise vehicle on hoist, and disconnect the fuel pressure line at the fuel pump module.
Install special 5/16 fuel line adapter tool #6539 or #6631 for 3/8 fuel lineadapter, between the disconnected fuel line
and the fuel pump module.
Attach a fuel pressure test gauge to the T fitting on tool #6539 or #6631.
Ignition on, engine not running.
With the scan tool, actuate the ASD FuelSystem test and observe the fuel pressure gauge.
NOTE: Fuel pressure specification is 379 KPa +/- 14 KPa (55 psi +/- 2 psi).
CAUTION: Stop All Actuations.
Is the fuel pressure within specification?
Ye s>>
Repair or replace fuel supply line as necessary.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 4
4.FUEL PUMP INLET STRAINER
Turn the ignition off.
WARNING: The fuel system is under a constant pressure (even with the engineoff). Before testing or ser-
vicing any fuel system hose, fitting or line, the fuel system pressure mustbe released.
Remove the Fuel Pump Module and inspect the Fuel Inlet Strainer.
Is the Fuel Inlet Strainer plugged?
Ye s>>
Replace the Fuel Pump Inlet Strainer.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 5

2.CHECKING FUEL PRESSURE
Turn the ignition off.
WARNING: The fuel system is under a constant pressure (even with the engineoff). Before testing or ser-
vicing any fuel system hose, fitting or line, the fuel system pressure mustbe released.
Install a fuel pressure gauge.
Ignition on, engine not running.
With the scan tool, actuate the ASD FuelSystem test and observe the fuel pressure gauge.
NOTE: Fuel pressure specification is 379 KPa +/- 14 KPa (55 psi +/- 2 psi).
Choose a conclusion that best matches your fuel pressure reading.
Within Specification
Go To 3
Above Specification
Replace the fuel filter/pressure regulator.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
CAUTION: Stop All Actuations.
3.O2 SENSOR OPERATION
Ignition on, engine not running.
With the scan tool, monitor all of the O2 Sensor voltage readings.
Is the voltage above 4.5 volts for all of the O2 Sensors?
Ye s>>
Go To 4
No>>
Go To 11
4.O2 SENSOR HEATER OPERATION
Turn the ignition off.
NOTE: Wait a minimum of 10 minutes to allow the O2 Sensor to cool down before continuing the test. Allow
the O2 Sensor voltage to stabilize between 4 and 5 volts.
Ignition on, engine not running.
With the scan tool, actuate the O2 Heater Test.
With the scan tool, monitor O2 Sensor voltage for at least 2 minutes.
Does the voltage stay above 4.5 volts?
Ye s>>
Replace the O2 Sensor that had the voltage reading above 4.5 volts.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 5
5.EVAP SYSTEM OPERATION
NOTE: The engine must be at operating temperature and in closed loop to perform this test.
Start the engine.
WARNING: When the engine is operating, do not stand in a direct line with thefan. Do not put your hands
near the pulleys, belts, or fan. Do not wear loose clothing.
Allow the engine to reach normal operating temperature.
With the scan tool select System Tests, perform the Purge Vapors Test.
Observe the Short Term Adaptive value and press 3 to flow.

2.CHECKING FUEL PRESSURE
WARNING: The fuel system is under a constant pressure (even with the engineoff). Before testing or ser-
vicing any fuel system hose, fitting or line, the fuel system pressure mustbe released.
Install a fuel pressure gauge.
Ignition on, engine not running.
With the scan tool, actuate the ASD FuelSystem test and observe the fuel pressure gauge.
NOTE: Fuel pressure specification is 379 KPa +/- 14 KPa (55 psi +/- 2 psi).
Turn the ignition off.
CAUTION: Stop All Actuations.
Choose a conclusion that best matches your fuel pressure reading.
Below Specification
Go To 3
Within Specification
Go To 6
Above Specification
Replace the fuel filter/pressure regulator.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
3.RESTRICTED FUEL SUPPLY LINE
Turn the ignition off.
WARNING: The fuel system is under a constant pressure (even with the engineoff). Before testing or ser-
vicing any fuel system hose, fitting or line, the fuel system pressure mustbe released.
Raise vehicle on hoist, and disconnect the fuel pressure line at the fuel pump module.
Install special 5/16 fuel line adapter tool #6539 or #6631 for 3/8 fuel lineadapter, between the disconnected fuel line
and the fuel pump module.
Attach a fuel pressure test gauge to the T fitting on tool #6539 or #6631.
Ignition on, engine not running.
With the scan tool, actuate the ASD FuelSystem test and observe the fuel pressure gauge.
NOTE: Fuel pressure specification is 379 KPa +/- 14 KPa (55 psi +/- 2 psi).
CAUTION: Stop All Actuations.
Is the fuel pressure within specification?
Ye s>>
Repair or replace fuel supply line as necessary.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 4
4.FUEL PUMP INLET STRAINER
Turn the ignition off.
WARNING: The fuel system is under a constant pressure (even with the engineoff). Before testing or ser-
vicing any fuel system hose, fitting or line, the fuel system pressure mustbe released.
Remove the Fuel Pump Module and inspect the Fuel Inlet Strainer.
Is the Fuel Inlet Strainer plugged?
Ye s>>
Replace the Fuel Pump Inlet Strainer.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 5

2.CHECKING FUEL PRESSURE
Turn the ignition off.
WARNING: The fuel system is under a constant pressure (even with the engineoff). Before testing or ser-
vicing any fuel system hose, fitting or line, the fuel system pressure mustbe released.
Install a fuel pressure gauge.
Ignition on, engine not running.
With the scan tool, actuate the ASD FuelSystem test and observe the fuel pressure gauge.
NOTE: Fuel pressure specification is 379 KPa +/- 14 KPa (55 psi +/- 2 psi).
Choose a conclusion that best matches your fuel pressure reading.
Within Specification
Go To 3
Above Specification
Replace the fuel filter/pressure regulator.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
CAUTION: Stop All Actuations.
3.O2 SENSOR OPERATION
Ignition on, engine not running.
With the scan tool, monitor all of the O2 Sensor voltage readings.
Is the voltage above 4.5 volts for all of the O2 Sensors?
Ye s>>
Go To 4
No>>
Go To 11
4.O2 SENSOR HEATER OPERATION
Turn the ignition off.
NOTE: Wait a minimum of 10 minutes to allow the O2 Sensor to cool down before continuing the test. Allow
the O2 Sensor voltage to stabilize between 4 and 5 volts.
Ignition on, engine not running.
With the scan tool, actuate the O2 Heater Test.
With the scan tool, monitor O2 Sensor voltage for at least 2 minutes.
Does the voltage stay above 4.5 volts?
Ye s>>
Replace the O2 Sensor that had the voltage reading above 4.5 volts.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 5
5.EVAP SYSTEM OPERATION
NOTE: The engine must be at operating temperature and in closed loop to perform this test.
Start the engine.
WARNING: When the engine is operating, do not stand in a direct line with thefan. Do not put your hands
near the pulleys, belts, or fan. Do not wear loose clothing.
Allow the engine to reach normal operating temperature.
With the scan tool select System Tests, perform the Purge Vapors Test.
Observe the Short Term Adaptive value and press 3 to flow.

NOTE: Lightly tap the bottom of the spark plug on a solid surface. The electrode in the spark plug should
not move.
Does the Spark Plug have any problems?
Ye s>>
Replace the Spark Plug.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 6
6.FUEL PRESSURE CHECK
WARNING: The fuel system is under a constant pressure (even with the engineoff). Before testing or ser-
vicing any fuel system hose, fitting or line, the fuel system pressure mustbe released. Failure to follow
these instructions can result in personal injury or death.
Install a fuel pressure gauge to the fuel rail.
Start the engine and observe the fuel pressure reading.
NOTE: Fuel pressure specification is 407 KPa +/- 34 KPa (59 psi +/- 5 psi).
Choose a conclusion that best matches your fuel pressure reading.
Within Specification
Go To 7
Below Specification
Go To 12
Above Specification
Replace the fuel filter/pressure regulator.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
7.FUEL PRESSURE LEAK DOWN
NOTE: Before continuing visually and physically inspect the fuel delivery system for external leaks or dam-
age. Repair / replace as necessary.
Turn the ignition off.
WARNING: The fuel system is under a constant pressure (even with the engineoff). Before testing or ser-
vicing any fuel system hose, fitting or line, the fuel system pressure mustbe released. Failure to follow
these instructions can result in personal injury or death
Installspecialtool6539(5/16
)or6631(3/8) fuel line adapter.
Install the fuel pressure gauge.
Start the engine and allow the fuel system to reach maximum pressure.
Turn the ignition off.
NOTE: Fuel specification is 407 KPa +/- 34 KPa (59 psi +/- 5 psi).
Using special tool C4390, Hose Clamp Pliers, pinch the rubber fuel line between the fuel pressure gauge and the
engine.
Monitor the fuel pressure gauge for a minimum of 5 minutes.
NOTE: The pressure should not fall below 241 KPa (35 psi)
Does the pressure fall below the listed specification?
Ye s>>
Replace the leaking Fuel Injector(s).
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 8

8.FUEL INJECTOR OPERATION
WARNING: The fuel system is under a constant pressure (even with the engineoff). Before testing or ser-
vicing any fuel system hose, fitting or line, the fuel system pressure mustbe released. Failure to follow
these instructions can result in personal injury or death.
CAUTION: After each Fuel Injector actuation, start the engine to clean thecylinder of fuel. Failure to do so
could cause engine damage.
Remove special tool C4390.
Start the engine and allow the fuel pressure to reach maximum pressure.
Ignition on, engine not running.
Using the scan tool, actuate the Fuel Injector for the cylinder that indicated the misfire.
Monitor the fuel pressure gauge.
Does the fuel pressure gauge indicate a drop in fuel pressure?
Ye s>>
Go To 9
No>>
Go To 10
NOTE: Turn the ignition off, remove the Fuel Pressure gauge, and connect the fuel lines before continuing.
9.ENGINE MECHANICAL
Check for any of the following conditions/mechanical problems.
ENGINE VACUUM - must be at least 13 inches in neutral
ENGINE VALVE TIMING - must be within specifications
ENGINE COMPRESSION - must be within specifications
ENGINE EXHAUST SYSTEM - must be free of any restrictions or leaks.
ENGINE PCV SYSTEM - must flow freely
TORQUE CONVERTER STALL SPEED - must be within specifications
POWER BRAKE BOOSTER - no internal vacuum leaks
FUEL - must be free of contamination
CAM LOBES - must not be worn excessively
CYLINDER LEAKAGE TEST - must be within specifications
VALVE SPRINGS - cannot be weak or broken
Are there any engine mechanical problems?
Ye s>>
Repair as necessary.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 17
10.FUEL INJECTOR
Turn the ignition off.
Disconnect the Fuel Injector harness connector of the misfiring cylinder.
Ignition on, engine not running.
NOTE: When a Misfire is detected for a particular cylinder, the PCM will shut down that cylinders Injector
Control circuit.
With the scan tool, erase DTCs.
Using a 12-volt test light connected to 12-volts, probe the Injector Control circuit.
With the scan tool, actuate the Fuel Injector.
Does the test light blink/flicker?
Ye s>>
Replace the Fuel Injector.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 11