Page 1009 of 5267
6.PCM
NOTE: Before continuing, check the PCM harness connector terminals for corrosion, damage, or terminal
push out. Repair as necessary.
Using the schematics as a guide, inspect the wire harness and connectors. Pay particular attention to all Power and
Ground circuits.
Were there any problems found?
Ye s>>
Repair as necessary.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Replace and program the Powertrain Control Module per Service Information.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
Page 1010 of 5267
P0138-O2 SENSOR 1/2 CIRCUIT HIGH
For a complete wiring diagramRefer to Section 8W.
Page 1011 of 5267

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
(K141) O2 SENSOR 1/2 SIGNAL SHORTED TO VOLTAGE
(K141) O2 SENSOR 1/2 SIGNAL CIRCUIT OPEN
(K141) O2 SENSOR 1/2 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
Page 1012 of 5267
3.(K141) O2 SENSOR 1/2 SIGNAL CIRCUIT SHORTED TO VOLTAGE
Turn the ignition off.
Disconnect the PCM harness connectors.
Ignition on, engine not running.
Using a 12-volt test light connected to ground, probe the (K141) O2
Sensor 1/2 Signal circuit at the 1/2 O2 Sensor harness connector.
Does the test light illuminate brightly?
Ye s>>
Repair the short to voltage in the (K141) O2 Sensor 1/2
Signal circuit.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to
9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 4
4.(K141) O2 SENSOR 1/2 SIGNAL CIRCUIT OPEN
Turn the ignition off.
Measure the resistance of the (K141) O2 Sensor 1/2 Signal circuit from
the 1/2 O2 Sensor harness connector to the PCM harness connector.
Is the resistance below 5.0 ohms?
Ye s>>
Go To 5
No>>
Repair the open in the (K141) O2 Sensor 1/2 Signal circuit.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to
9 - ENGINE - STANDARD PROCEDURE)
Page 1013 of 5267
5.(K141) O2 SENSOR 1/2 SIGNAL CIRCUIT SHORTED TO THE (K344) O2 HEATER RELAY OUTPUT CIRCU
IT
Measure the resistance between the (K141) O2 Sensor 1/2 Signal cir-
cuit and the (K344) O2 Heater Relay Output circuit at the O2 Sensor
harness connector.
Istheresistancebelow100ohms?
Ye s>>
Repair the short between the (K141) O2 Sensor 1/2 Signal
circuit and the (K344) O2 Heater Relay Output circuit.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to
9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 6
6.(K900) SENSOR GROUND CIRCUIT OPEN
Measure the resistance of the (K900) Sensor ground circuit from the 1/2
O2 Sensor harness connector to the PCM harness connector.
Is the resistance below 5.0 ohms?
Ye s>>
Go To 7
No>>
Repair the open in the (K900) Sensor ground circuit.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to
9 - ENGINE - STANDARD PROCEDURE)
Page 1014 of 5267

7.(K344) O2 SENSOR HEATER RELAY OUTPUT CIRCUIT OPEN
Connect the PCM harness connectors.
Ignition on, engine not running.
With the scan tool actuate the O2 Heater Test.
Measure the voltage on the (K344) O2 Sensor Heater Relay Output cir-
cuit.
Is the voltage above 11.0 volts?
No>>
Repair the open in the (K344) O2 Sensor Heater Relay Out-
put circuit.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to
9 - ENGINE - STANDARD PROCEDURE)
Ye s>>
Go To 8
8.(Z1) O2 SENSOR HEATER GROUND CIRCUIT OPEN
Turn the ignition off.
Disconnect the PCM harness connectors.
Measure the resistance between ground and the (Z1) O2 Sensor Heater
ground circuit from the 1/2 O2 Sensor harness connector.
Is the resistance below 5.0 ohms?
Ye s>>
Go To 9
No>>
Repair the open in the (Z1) O2 Sensor Heater ground cir-
cuit.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to
9 - ENGINE - STANDARD PROCEDURE)
9.PCM
NOTE: Before continuing, check the PCM harness connector terminals for corrosion, damage, or terminal
push out. Repair as necessary.
Using the schematics as a guide, inspect the wire harness and connectors. Pay particular attention to all Power and
Ground circuits.
Were there any problems found?
Ye s>>
Repair as necessary.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Replace and program the Powertrain Control Module per Service Information.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
Page 1015 of 5267
P0139-O2 SENSOR 1/2 SLOW RESPONSE
For a complete wiring diagramRefer to Section 8W.
Page 1016 of 5267

Theory of Operation
The downstream O2 Sensor is located in the exhaust path behind the catalytic converter, is monitored for proper
response to assure optimum catalytic converter efficiency. The downstream O2 response monitor is intended to
diagnose a downstream O2 sensor that is not moving or stuck in a voltage window and to insure accurate infor-
mation for catalyst monitor diagnosis.
When Monitored:
Start engine. Allow engine to idle. For 1st part of test, if limits are exceeded, test passes. If not, 2nd part of
test runs. amb/batt temp >44°F, Baro >22.13
H2O, battery >10.5 volts, MAP >11.79 & <18.15H2O, RPM
>1350 & <2200 and vss >50 and <65.
Set Condition:
The oxygen sensor signal voltage is switching from below 0.39 of a volt to above 0.58 of a volt and back fewer
times than required.
Possible Causes
EXHAUST LEAK
(K141) O2 1/2 SIGNAL CIRCUIT
(K900) SENSOR GROUND CIRCUIT
O2 SENSOR
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE -
DIAGNOSIS AND TESTING).
Diagnostic Test
1.ACTIVE DTC
NOTE: Check for contaminants that may have damaged the O2 Sensor: contaminated fuel, unapproved sil-
icone, oil and coolant.
Ignition on, engine not running.
NOTE: After the repairs have been made, verify proper O2 Sensor operation.If all the O2 Sensor voltage
readings have not returned to normal, follow the diagnostic procedure forthe remaining O2 Sensors.
With a scan tool, read DTCs.
Is the DTC active at this time?
Ye s>>
Go To 2
No>>
Refer to the INTERMITTENT CONDITION Symptom. (Refer to 9 - ENGINE - STANDARD PROCE-
DURE)
2.EXHAUST LEAK
Start the engine.
Inspect the exhaust system for leaks between the engine and the O2 Sensors.
Are there any exhaust leaks?
Ye s>>
Repair or replace the leaking exhaust parts as necessary.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 3