Isuzu N-Series. Manual - part 782

Engine Control System (4JH1) 6E-179

P1120 (Symptom Code E) (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 supplies 5 volts to the APP sensor on the 5
volts reference circuit. The ECM also provides a
ground on the low reference circuit. The APP sensor
provides a signal to the ECM on the APP sensor signal
circuit, which is relative to the position changes of the
accelerator pedal angle.
The idle switch is also mounted on the accelerator
pedal bracket. The idle switch is a normally closed
type switch. When the accelerator pedal is released,
the idle switch signal to the ECM is low voltage.
If the ECM detects the APP sensor signal and the idle
switch out of correlation, this DTC will set.

Condition for Running the DTC
The ignition switch is ON.

Condition for Setting the DTC

• The ECM detects APP sensor angle is more than

18%, then the idle switch is ON for longer than 0.6
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 E) (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

Turn ON the ignition, with the engine OFF.

Observe the Idle Switch parameter with the
Tech 2 while depressing and releasing the
accelerator pedal.

Does the Tech 2 indicate OFF when the accelerator
pedal is depressed and ON when the accelerator
pedal is released?

—

Go to Step 5

Go to Step 3

Turn OFF the ignition.

Disconnect the idle switch harness connector.

Turn ON the ignition, with the engine OFF.

Observe the Idle Switch parameter with the
Tech 2 while momentarily jumping 3-amp
fused jumper wire across the idle switch
harness connector between pins 1 and 2 of
the B-340 connector.

Does the Tech 2 indicate ON when the circuit is
jumpered and OFF when the circuit is not
jumpered?

—

Go to Step 7

Go to Step 4

6E-180 Engine Control System (4JH1)

Test the switch signal circuit between the
engine control module (ECM) (pin 69 of B-234
connector) and the idle switch (pin 1 of B-340
connector) for a short to ground.

Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 10

Go to Step 9

Turn ON the ignition, with the engine OFF.

Fully depress and release the accelerator
pedal many times while observing the
Accelerator Pedal Position (APP) parameter
with the Tech 2.

Is the APP parameter smoothly changed within 0 to
100% when the accelerator pedal is depressed and
released?

—

Go to Diagnostic

Aids

Go to Step 6

Turn OFF the ignition.

Disconnect the APP sensor harness
connector.

Inspect for an intermittent, for poor connection
and corrosion at the harness connector of the
APP sensor (pin 1 of B-80 connector).

Disconnect the ECM harness connector.

Inspect for an intermittent, for a poor
connection and corrosion on the APP sensor
circuit at the harness connector of the ECM
(pin 49 of B-234 connector).

Test for high resistance on the APP sensor
low reference circuit.

Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 10

Go to Step 8

Replace the idle switch. Refer to Idle Switch
Replacement in this section.

Did you complete the replacement?

—

Go to Step 10

—

Replace the accelerator pedal position sensor.
Refer to Accelerator Pedal Position Sensor
Replacement in this section.

Did you complete the replacement?

—

Go to Step 10

—

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 10

—

Step

Action

Value(s)

Yes

Engine Control System (4JH1) 6E-181

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.

Observe the Idle Switch and APP parameter
while fully depressing and releasing the
accelerator pedal many times.

Does the Tech 2 indicate the APP parameter
smoothly changed within 0 to 100% when the
accelerator pedal is depressed and released, then
Tech 2 indicate the Idle Switch parameter OFF
when the accelerator pedal is depressed and ON
when the accelerator pedal is released?

—

Go to Step 11

Go to Step 2

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-182 Engine Control System (4JH1)

P1173 (Symptom Code 3) (Flash Code 22)

Circuit Description
The engine coolant temperature (ECT) sensor is
installed to the thermostat housing. The ECT sensor is
a variable resistor. The ECT sensor has a signal circuit
and a low reference circuit. The ECT sensor measures
the temperature of the engine coolant. The engine
control module (ECM) supplies 5 volts to the ECT
signal circuit and a ground for the ECT low reference
circuit. When the ECT sensor is cold, the sensor
resistance is high. When the engine coolant
temperature increases, the sensor resistance
decreases. With high sensor resistance, the ECM
detects a high voltage on the ECT signal circuit. With
lower sensor resistance, the ECM detects a lower
voltage on the ECT signal circuit. If the ECM detects
an excessive high coolant temperature, this DTC will
set.

Condition for Running the DTC

• The ignition switch is ON.

Condition for Setting the DTC

• The ECM detects an excessive high coolant

temperature for longer than 30 seconds.

Action Taken When the DTC Sets

• The ECM does not illuminate the malfunction

indicator lamp (MIL) when the diagnostic runs and
fails.

Condition for Clearing the DTC

• 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.

• After starting the engine, the ECT should rise

steadily to about 80

°C – 85°C (176°F – 185°F)

then stabilize when the thermostat opens.

• Use the Temperature vs. Resistance table to test

the ECT sensor at various temperature levels to
evaluate the possibility of a skewed sensor. A
skewed sensor could result in poor driveability
concerns.

Notice:

• This DTC is caused by an engine overheat

condition (i.e. low engine coolant level). Since this
DTC does not illuminate any lamps, clear the DTC
and ensure there are no signs of engine damage.
Excessive engine overheat may damage internal
engine components.

DTC P1173 (Symptom Code 3) (Flash Code 22)

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 and wait until engine is fully
warms upped.

Observe the Engine Coolant Temperature
(ECT) parameter with the Tech 2.

Is the ECT parameter more than the specified
value?

100

°C (212°F)

Go to Step 4

Go to Step 3

Ask the driver if overheat is caused by low engine
coolant level, etc.
If engine overheat has experienced, the engine
must be inspected and repaired as necessary.

Did you complete the action?

—

Go to Step 6

—

Test the ECT sensor at various temperature
levels to evaluate the possibility if a skewed
sensor.

Replace the ECT sensor as necessary.

Did you find and correct the condition?

—

Go to Step 6

Go to Step 5

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