Isuzu Trooper (1998-2002 year). Manual - part 306

6E–137

ENGINE DRIVEABILITY AND EMISSIONS

Disconnect the component while observing the TP
sensor display on the Tech 2.  If the reading
changes drastically when this component is

disconnected, replace the component that affected
the reading.

DTC P0123 – TP Sensor Circuit High Voltage

 

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. Ignition “ON,” engine “OFF.”
2. With the throttle closed, observe the “TP Sensor”

display on the Tech 2.

Is the “TP Sensor” above the specified value?

4.88 V

Go to 

Step 4

Go to 

Step 3

3

1. Ignition “ON,” engine “OFF.”
2. Review and record Tech 2 Failure Records data.
3. Operate the vehicle within Failure Records

conditions as noted.

4. Using a Tech 2, monitor “Specific DTC” info for DTC

P0123.

Does the Tech 2 indicate DTC P0123 failed.

—

Go to 

Step 4

Refer to

Diagnostic

Aids

4

1. Disconnect the TP sensor electrical connector.
2. Observe the “TP Sensor” display on the Tech 2.

Is the “TP Sensor” near the specified value?

0 V

Go to 

Step 5

Go to 

Step 6

5

Probe the sensor ground circuit at the TP sensor
harness connector with a test light connected to B+.

Is the test light “ON?”

—

Go to 

Step 7

Go to 

Step 10

6

1. Ignition “OFF,” disconnect the PCM.
2. Ignition “ON,” engine “OFF.”
3. Check for a short to voltage on the TP sensor signal

circuit.

4. If the TP sensor signal circuit is shorted, repair it as

necessary.

Was the TP sensor signal circuit shorted?

—

Verify repair

Go to 

Step 12

7

1. Ignition “ON.”
2. Monitor the “TP Sensor” Tech 2 display while

disconnecting each of the components that share
the 5 volt reference “A” circuit (one at a time).

3. If the “TP Sensor” Tech 2 display changes, replace

the component that caused the display to change
when disconnected.

Does disconnecting any of these components cause
the “TP Sensor” display to change?

—

Verify repair

Go to 

Step 8

8

1. Ignition “OFF,” disconnect the PCM.
2. Ignition “ON,” engine “OFF.”
3. Check for a short to B+ on the 5 volt reference “A”

circuit.

4. If the 5 volt reference “A” circuit is shorted, repair it

as necessary.

Was the 5 volt reference “A” circuit shorted?

—

Verify repair

Go to 

Step 9

9

Check for poor electrical connections at the TP sensor
and replace terminals if necessary.

Did any terminals require replacement?

—

Verify repair

Go to 

Step 11

6E–138

ENGINE DRIVEABILITY AND EMISSIONS

DTC P0123 – TP Sensor Circuit High Voltage

  (Cont'd)

Step

No

Yes

Value(s)

Action

10

1. Ignition “OFF.”
2. Disconnect the PCM, and check for an open sensor

ground circuit to the TP sensor.

3. If a problem is found, repair it as necessary.

Was the sensor ground circuit to the TP sensor open?

—

Verify repair

Go to 

Step 12

11

Replace the TP sensor.

Is the action complete?

—

Verify repair

—

12

Replace the PCM.

IMPORTANT: The replacement PCM must be
programmed. Refer to 

UBS 98model year Immobilizer

Workshop Manual.

Is the action complete?

—

Verify repair

—

6E–139

ENGINE DRIVEABILITY AND EMISSIONS

Diagnostic Trouble Code (DTC) P0131 HO2S Circuit Low Voltage Bank 1 

Sensor 1

060RW236

Circuit Description

The powertrain control module (PCM) supplies a 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 350 mV.  The oxygen sensor varies the
voltage within a range of about 1000 mV when the
exhaust is rich, down through about 10 mV when 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 1 voltage
remains excessively low for an extended period of time,
DTC P0131 will be set.

Conditions for Setting the DTC

D

No related DTCs.

D

Vehicle is operating in “closed loop.”

D

Engine coolant temperature is above 60

°

C (140

°

F).

D

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

D

Throttle angle is between 3% and 19%.

D

Bank 1 HO2S 1 signal voltage remains below 22 mV
during normal “closed loop” operation for a total of 77
seconds over a 90-second period of time.

Action Taken When the DTC Sets

D

The PCM will illuminate the malfunction indicator lamp
(MIL) the first time the fault is detected.

D

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

D

“Open loop” fuel control will be in effect.

Conditions for Clearing the MIL/DTC

D

DTC P0131 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

Heated oxygen sensor wiring – The sensor pigtail may
be routed incorrectly and contacting the exhaust
system.

D

Poor PCM to engine block grounds.

D

Fuel pressure – The system will go lean if pressure is
too low.  The PCM can compensate for some
decrease.  However, If fuel pressure is too low, a DTC
P0131 may be set.  Refer to 

Fuel System Diagnosis.

D

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

D

Vacuum leaks – Check for disconnected or damaged
vacuum hoses and for vacuum leaks at the intake
manifold, throttle body, EGR system, and PCV system.

D

Exhaust leaks – An exhaust leak may cause outside air
to be pulled into the exhaust gas stream past the

6E–140

ENGINE DRIVEABILITY AND EMISSIONS

HO2S, causing the system to appear lean.  Check for
exhaust leaks that may cause a false lean condition to
be indicated.

D

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

D

Fuel contamination – Water, even in small amounts,
can be delivered to the fuel injectors.  The water can
cause a lean exhaust to be indicated.  Excessive
alcohol in the fuel can also cause this condition.  Refer
to 

Fuel System Diagnosis for the procedure to check

for fuel contamination.

D

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

Test Description

Number(s) below refer to step numbers on the diagnostic
chart.
3. DTC P0131 failing during operation may indicate a

condition described in the “Diagnostic Aids” above.
If the DTC P0131 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 P0131 –HO2S Circuit Low Voltage Bank 1 Sensor 1

 

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 1 voltage.

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

22 mV

Go to 

Step 4

Go to 

Step 3

3

1. Ignition “ON,” engine “OFF,” review and record Tech

2 Failure Records data and note parameters.

2. Operate the vehicle within Failure Records

conditions as noted.

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

P0131 until the DTC P0131 test runs.

Note test result.

Does Tech 2 indicate DTC P0131 failed this ignition?

—

Go to 

Step 4

Refer to

Diagnostic

Aids

4

1. Turn the ignition “OFF.”
2. Disconnect the PCM.
3. Check the Bank 1 HO2S 1 high and low circuits for a

short to ground or a short to the heater ground
circuit.

Are the Bank 1 HO2S 1 signal circuits shorted to
ground?

—

Go to 

Step 5

Go to 

Step 6

5

Repair the Bank 1 HO2S 1 signal circuit.

Is the action complete?

—

Verify repair

—

6

1. Turn the ignition “OFF,” HO2S 1 and PCM

disconnected.

2. Check for continuity between the high and low

signal circuits.

Was there continuity between the high and low circuits?

—

Go to 

Step 7

Go to 

Step 8

7

Repair the short between the high and low circuits.

Is the action complete?

—

Verify repair

—