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Isuzu N-Series. Service manual - part 780

Engine Control System (4JH1) 6E-171

Reconnect all previously disconnected
harness connector(s).

Clear the DTCs with the Tech 2.

Turn OFF the ignition for 30 seconds.

Turn ON the ignition, with the engine OFF.

Fully depress and release the accelerator
pedal many times.

Monitor the DTC Information with the Tech 2.

Did the DTC fail this ignition?

—

Go to Step 3

Go to Step 12

Observe the DTC Information with the Tech 2.

Are there any DTCs that you have not diagnosed?

—

Go to Diagnostic

Trouble Code (DTC)

List

System OK

Step

Action

Value(s)

Yes

6E-172 Engine Control System (4JH1)

P1120 (Symptom Code 7) (Flash Code 21)

Circuit Description
The accelerator pedal position (APP) sensor is
mounted on the accelerator pedal bracket. The engine
control module (ECM) uses the APP sensors to
determine the amount of acceleration or deceleration
desired by the person driving the vehicle via the fuel
injection control. The APP sensor has the following
circuits.

• 5 volts reference circuit
• Low reference circuit
• APP sensor signal circuit

The ECM provides 5 volts reference voltage through
the reference circuit to the APP sensor. The ECM
monitors the voltage on the 5 volts reference circuit. If
the ECM detects an excessively high APP sensor 5
volts reference voltage, this DTC will set.

Condition for Running the DTC

• The ignition switch is ON.

Condition for Setting the DTC
The ECM detects that the APP sensor voltage supply
circuit is more than 5.2 volts for 0.5 seconds.

Action Taken When the DTC Sets

• The ECM illuminates the malfunction indicator

lamp (MIL) when the diagnostic runs and fails.

• The ECM uses an engine speed substitution of

1400 RPM.

Condition for Clearing the MIL/DTC

• The ECM turns OFF the MIL when the diagnostic

runs and does not fail.

• A history DTC clears after 40 consecutive driving

cycles without a fault. Or clear with the Tech 2.

Diagnostic Aids

• If an intermittent condition is suspected, refer to

Intermittent Conditions in this section.

DTC P1120 (Symptom Code 7) (Flash Code 21)
Schematic Reference: Engine Controls Schematics
Connector End View Reference: Engine Controls
Connector End Views or Engine Control Module (ECM)
Connector End Views

Step

Action

Value(s)

Yes

Did you perform the Diagnostic System Check –
Engine Controls?

—

Go to Step 2

Go to Diagnostic

System Check –

Engine Controls

Install the Tech 2.

Turn OFF the ignition for 30 seconds.

Start the engine.

Monitor the Diagnostic Trouble Code (DTC)
Information with the Tech 2.

Does the DTC fail this ignition?

—

Go to Step 3

Go to Diagnostic

Aids

Test the 5 volts reference circuit between the
engine control module (ECM) (pin 57 of B-234
connector) and the accelerator pedal position
(APP) sensor (pin 3 of B-80 connector) for a
short to battery or ignition voltage.

Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 5

Go to Step 4

Important:
Replacement ECM must be programmed.

Replace the ECM. Refer to Engine Control Module
(ECM) Replacement in this section.

Did you complete the replacement?

—

Go to Step 5

—

Engine Control System (4JH1) 6E-173

Reconnect all previously disconnected
harness connector(s).

Clear the DTCs with the Tech 2.

Turn OFF the ignition for 30 seconds.

Turn ON the ignition, with the engine OFF.

Fully depress and release the accelerator
pedal many times.

Monitor the DTC Information with the Tech 2.

Did the DTC fail this ignition?

—

Go to Step 3

Go to Step 6

Observe the DTC Information with the Tech 2.

Are there any DTCs that you have not diagnosed?

—

Go to Diagnostic

Trouble Code (DTC)

List

System OK

Step

Action

Value(s)

Yes

6E-174 Engine Control System (4JH1)

P1120 (Symptom Code 9) (Flash Code 21)

• 5 volts reference circuit
• Low reference circuit
• APP sensor signal circuit

Condition for Running the DTC

• The ignition switch is ON.

Condition for Setting the DTC

• The ECM detects that the APP sensor voltage

supply circuit is less than 4.6 volts for 0.5 seconds.

Action Taken When the DTC Sets

• The ECM illuminates the malfunction indicator

lamp (MIL) when the diagnostic runs and fails.

• The ECM uses an engine speed substitution of

1400 RPM.

Condition for Clearing the MIL/DTC

• The ECM turns OFF the MIL when the diagnostic

runs and does not fail.

• A history DTC clears after 40 consecutive driving

cycles without a fault. Or clear with the Tech 2.

Diagnostic Aids

• If an intermittent condition is suspected, refer to

Intermittent Conditions in this section.

Test Description
The number below refers to the step number on the
diagnostic table.
3. If the APP sensor 5 volts reference circuit between
the ECM and the sensor is normal, the sensor signal
low DTC P1120 (Symptom Code 2) will set.

DTC P1120 (Symptom Code 9) (Flash Code 21)
Schematic Reference: Engine Controls Schematics
Connector End View Reference: Engine Controls
Connector End Views or Engine Control Module (ECM)
Connector End Views

Step

Action

Value(s)

Yes

Did you perform the Diagnostic System Check –
Engine Controls?

—

Go to Step 2

Go to Diagnostic

System Check –

Engine Controls

Install the Tech 2.

Turn OFF the ignition for 30 seconds.

Start the engine.

Monitor the Diagnostic Trouble Code (DTC)
Information with the Tech 2.

Does the DTC fail this ignition?

—

Go to Step 3

Go to Diagnostic

Aids

Turn OFF the ignition.

Disconnect the accelerator pedal position
(APP) sensor harness connector.

Turn ON the ignition, with the engine OFF.

Depress and release the accelerator pedal.

Monitor the DTC Information with the Tech 2.

Does DTC P1120 (Symptom Code 2) set, but not
DTC P1120 (Symptom Code 9)?

—

Go to Step 5

Go to Step 4

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