Isuzu Trooper (1998-2002 year). Manual - part 451

6E–174

6VE1 3.5 ENGINE DRIVEABILITY AND EMISSIONS

DTC P0137 –HO2S Circuit Low Voltage Bank 1 Sensor 2

  (Cont'd)

Step

No

Yes

Value(s)

Action

7

Repair the short between the high and low circuits.

Is the action complete?

—

Verify repair

—

8

1. Ignition “OFF”.
2. Reconnect the PCM, leave HO2S 2 disconnected.
3. Ignition “ON”.

Does the Tech 2 indicate Bank 1 HO2S 2 voltage near
the specified value?

425-475 mV

Check the

Procedure

Refer to

Diagnostic

Aids

Go to 

Step 9

9

Replace the PCM.

IMPORTANT: The replacement PCM must be
programmed.  Refer to 

On-Vehicle Service in

Powertrain Control Module and Sensors for
procedures.

And also refer to latest Service Bulletin.

Check to see if the Latest software is released or not.
And then Down Load the LATEST PROGRAMMED
SOFTWARE to the replacement PCM.

Is the action complete?

—

Verify repair

—

6E–175

6VE1 3.5L ENGINE DRIVEABILITY AND EMISSIONS

Diagnostic Trouble Code (DTC) 

P0138 HO2S Circuit High Voltage Bank 1 Sensor 2

D06RY00173

Circuit Description

The powertrain control module (PCM) supplies bias
voltage of about 450 mV between the heated oxygen
sensor (HO2S) signal high and signal low circuits.  When
measured with a 10 megaohm digital voltmeter, this may
display as low as 320 mV.  The oxygen sensor varies the
voltage within a range of about 1000 mV when exhaust is
rich, down through about 10 mV when the exhaust is
lean.  The PCM constantly monitors the HO2S signal
during “closed loop” operation and compensates for a rich
or lean condition by decreasing or increasing injector
pulse width as necessary.  If the Bank 1 HO2S 2 voltage
remains excessively high for an extended period of time,
DTC P0138 will be set.

Conditions for Setting the DTC

D

No related DTCs.

D

Engine is operating in “closed loop”.

D

“Closed loop” commanded air/fuel ratio is between
14.5 and 14.8.

D

Engine coolant temperature is above 60

°

C (140

°

F).

D

Throttle angle is between 3% and 19%.

D

Bank 1 HO2S 2 signal voltage remains above 952 mV
during normal “closed loop” operation for a total of 106
seconds over a 125-second period of time.

OR

D

Bank 1 HO2S 2 signal voltage remains above 500 mV
during deceleration fuel cut–off mode operation for up
to 3 seconds.

Action Taken When the DTC Sets

D

The PCM will ON the MIL after second trip with
detected fault.

D

The PCM will store conditions which were present
when the DTC was set as Freeze Frame and in the
Failure Records data.

Conditions for Clearing the MIL/DTC

D

The PCM will turn the MIL “OFF” on the third
consecutive trip cycle during which the diagnostic has
been run and the fault condition is no longer present.

D

A history DTC P0138 will clear after 40 consecutive
warm-up cycles have occurred without a fault.

D

DTC P0138 can be cleared by using the Tech 2 “Clear
Info” function or by disconnecting the PCM battery
feed.

Diagnostic Aids

Check for the following conditions:

D

Fuel pressure – An excessively rich fuel mixture can
cause a DTC P0138 to be set.  Refer to 

Fuel System

Diagnosis.

D

Rich injector(s) – Perform “Injector Balance Test”.

D

Leaking injector – Refer to 

Fuel System Diagnosis.

6E–176

6VE1 3.5 ENGINE DRIVEABILITY AND EMISSIONS

D

Evaporative emissions (EVAP) canister purge – Check
for fuel saturation.  If full of fuel, check the canister
control and hoses.  Refer to 

Evaporative Emission

(EVAP) Control System.

D

MAF sensor –The system can go rich if the MAF
sensor signal indicates an engine airflow
measurement that is not correct. Disconnect the MAF
sensor to see if the rich condition is corrected.  If so,
replace the MAF sensor.

D

Check for a leak in fuel pressure regulator diaphragm
by checking the vacuum line to the regulator for the
presence of fuel.  There should be no fuel in the
vacuum line.

D

TP sensor – An intermittent TP sensor output will
cause the system to go rich, due to a false indication
of the engine accelerating.

D

Shorted Heated Oxygen Sensor (HO2S) – If the HO2S
is internally shorted the HO2S voltage displayed on the
Tech 2 will be over 1 volt.  Try disconnecting the
affected HO2S with the key “ON”, engine “OFF”.  If the
displayed HO2S voltage changes from over 1000 mV
to around 450 mV, replace the HO2S.  Silicon
contamination of the HO2S can also cause a high
HO2S voltage to be indicated.  This condition is
indicated by a powdery deposit on the portion of the
HO2S exposed to the exhaust stream.  If
contamination is noticed, replace the affected HO2S.

D

Open HO2S Signal Circuit of Faulty HO2S – A poor
connection or open in the HO2S signal circuit can
cause the DTC to set during deceleration fuel mode.
An HO2S which is faulty and not allowing a full voltage
swing between the rich and lean thresholds can also
cause this condition.  Operate the vehicle while
monitoring the HO2S voltage with a Tech 2.  If the
HO2S voltage is limited within a range between
300 mV to 600 mV, check the HO2S signal and wiring
and associated terminal connections.

D

If none of the above conditions are present, replace the
affected HO2S.

Test Description

Number(s) below refer to the step number(s) on the
Diagnostic Chart.
3. DTC P0138 being set during deceleration fuel mode

operation may indicate a condition described in the
“Diagnostic Aids” above.  If the DTC P0138 test
passes while the Failure Records conditions are
being duplicated, an intermittent condition is
indicated.  

Reviewing the Failure Records vehicle mileage since the
diagnostic test last failed may help determine how often
the condition that caused the DTC to be set occurs.  This
may assist in diagnosing the condition.

DTC P0138 – HO2S Circuit  High Voltage Bank 1 Sensor 2

 

Step

Action

Value(s)

Yes

No

1

Was the “On-Board Diagnostic (OBD) System Check”
performed?

—

Go to 

Step 2

Go to 

OBD

System

Check

2

1. Install the Tech 2.
2. Run the engine at operating temperature.
3. Operate the vehicle within the parameters specified

under “Conditions for Setting the DTC” criteria
included in Diagnostic Support.

4. Using a Tech 2, monitor Bank 1 HO2S 2 voltage.

Does the Bank 1 HO2S voltage remain above the
specified value?

952 mV

(500 mV in

deceleration

fuel cutoff

mode)

Go to 

Step 4

Go to 

Step 3

3

1. Ignition “ON”, review and record Tech 2 Failure

Records data.

2. Operate the vehicle within Failure Records

conditions as noted.

3. Using a Tech 2, monitor “DTC” info for DTC P0138

until the DTC P0138 test runs.

4. Note the test result.

Does the Tech 2 indicate DTC P0138 failed this
ignition?

—

Go to 

Step 4

Check the

Procedure

Refer to

Diagnostic

Aids

4

1. Ignition “OFF”.
2. Disconnect Bank 1 HO2S 1.
3. Ignition “ON”.
4. At the HO2S Bank 1 Sensor 2 connector (PCM

side), use a DVM to measure voltages at the high
and low signal terminals.

Are the voltages above the specified range?

3-4 V

Go to 

Step 5

Go to 

Step 6

6E–177

6VE1 3.5L ENGINE DRIVEABILITY AND EMISSIONS

DTC P0138 – HO2S Circuit  High Voltage Bank 1 Sensor 2

  (Cont'd)

Step

No

Yes

Value(s)

Action

5

Repair short to voltage in the signal circuit.

Is the action complete?

—

Verify repair

—

6

1. Ignition “ON”, engine“OFF”.
2. At Bank 1 HO2S 2 connector (PCM side) jumper

both the HO2S high and low signal circuits (PCM
side) to ground.

3. Using a Tech 2, monitor Bank 1 HO2S 2 voltage.

Is Bank 1 HO2S 2 voltage below the specified value?

10 mV

Go to 

Step 7

Go to 

Step 8

7

1. Disconnect the jumpers to ground from Bank 1

HO2S 2 PCM–side connector.

2. With the HO2S 2 connector disconnected, monitor

BANK 1 HO2S 2 voltage.

Is the Bank 1 HO2S 2 voltage between the specified
values?

425-475 mV

Check the

Procedure

Refer to

Diagnostic

Aids

Go to 

Step 8

8

Replace the PCM.

IMPORTANT: The replacement PCM must be
programmed.  Refer to 

On-Vehicle Service in

Powertrain Control Module and Sensors for
procedures.

And also refer to latest Service Bulletin.

Check to see if the Latest software is released or not.
And then Down Load the LATEST PROGRAMMED
SOFTWARE to the replacement PCM.

Is the action complete?

—

Verify repair

—