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Isuzu engine C22NE / 22LE / 20LE. Manual - part 56

DRIVEABILITY AND EMISSIONS 6E1-53

VERIFYING VEHICLE REPAIR

Verification of vehicle repair will be more

comprehensive for vehicles with OBD system

diagnostics. Following a repair, the technician should

perform the following steps:

1. Review and record the customer complaint for the

DTC which has been diagnosed.

2. Clear DTC(s).

3. Operate the vehicle within conditions noted in the

customer complaint.

4. Monitor the DTC status information for the specific

DTC which has been diagnosed until the diagnostic

test associated with that DTC runs.

Following these steps are very important in verifying

repairs on OBD systems. Failure to follow these steps

could result in unnecessary repairs.

READING DIAGNOSTIC TROUBLE

CODES USING A SCAN TOOL

The procedure for reading diagnostic trouble code(s)

is to use a diagnostic Scan Tool. When reading

DTC(s), follow instructions supplied by tool

manufacturer.

Clearing Diagnostic Trouble Codes

Important: Do not clear DTCs unless directed to do

so by the service information provided for each

diagnostic procedure.

If the fault that caused the DTC to be stored into

memory has been corrected, the Diagnostic Executive

will begin to count the “warm-up” cycles with no further

faults detected, the DTC will automatically be cleared

form the ECM memory.

To clear Diagnostic Trouble Codes (DTCs), use the

diagnostic Scan Tool “clear DTCs” or “clear

information” function. When clearing DTCs follow

instructions supplied by the tool manufacturer.

When a Scan Tool is not available, DTCs can also be

cleared by disconnecting one of the following sources

for at least thirty (30) seconds.

Notice: To prevent system damage, the ignition key

must be OFF when disconnecting or reconnecting

battery power.
• The power source to the control module. Examples:

fuse, pigtail at battery ECM connectors etc.

• The negative battery cable. (Disconnecting the

negative battery cable will result in the loss of other

on-board memory data, such as preset radio

tuning).

6E1-54 DRIVEABILITY AND EMISSIONS

DTC MODES

There are 2 options available in the Scan Tool DTC

mode to display the enhanced information available.

After selecting DTC, the following menu appears:
• DTC Info
• Clear DTC Info
The following is a brief description of each of the sub

menus in DTC Info and Specific DTC. The order in

which they appear here is alphabetical and not

necessarily the way they will appear on the Scan Tool.

DTC Information Mode

Use the DTC info mode to search for a stored DTC

Refer to “Diagnostic Trouble Codes” list.

Clear DTC Information

The command for clearing DTC’s performs a

complete reset of the ECM.

PRIMARY SYSTEM-BASED

DIAGNOSTICS

PRIMARY SYSTEM-BASED

DIAGNOSTICS

There are primary system-based diagnostics which

evaluate system operation and its effect on vehicle

emissions. The primary system-based diagnostics are

listed below with a brief description of the diagnostic

function:

FUEL CONTROL HEATED OXYGEN

SENSORS (IF APPLICABLE)

The main function of the fuel control heated oxygen

sensors is to provide the control module with exhaust

stream oxygen content information to allow proper

fueling and maintain emissions within mandated

levels. After it reaches operating temperature, the

sensor will generate a voltage, inversely proportional

to the amount of oxygen present in the exhaust gases.

The control module uses the signal voltage from the

fuel control heated oxygen sensors while in “Closed

Loop” to adjust fuel injector pulse width. While in

“Closed Loop” , the ECM can adjust fuel delivery to

maintain an air/fuel ratio which allows the best

combination of emission control and driveability. The

fuel control heated oxygen sensors are also used to

determine catalyst efficiency.

HO2S HEATER

Heated oxygen sensors are used to minimize the

amount of time required for “Closed Loop” fuel control

to begin operation and to allow accurate catalyst

monitoring. The oxygen sensor heater greatly

decreases the amount of time required for fuel control

sensor (HO2S) to become active.

DRIVEABILITY AND EMISSIONS 6E1-55

ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK

16 15 14 13 12 11 10 9

8 7 6 5 4 3 2 1

HOT AT ALL TIMES

Fuse

Stop

10A

U/H

FUSE

BLOCK

Stop

Lamp

Engine

Control

Module

(ECM)

0.5G

B11

B/R

0.5

ECM

Grounds

(Engine Block)

Diag

Serial Data

1.25

Circuit Description

The on-board diagnostic system check is the starting

point for any driveability complaint diagnosis. Before

using this procedure, perform a careful visual/physical

check of the ECM and engine grounds for cleanliness

and tightness.

The on-board diagnostic system check is an organized

approach to identifying a problem created by an

electronic engine control system malfunction.

Diagnostic Aids

An intermittent may be caused by a poor connection,

rubbed-through wire insulation or a wire broken inside

the insulation. Check for poor connections or a

damaged harness. Inspect the ECM harness and

connectors for improper mating, broken locks,

improperly formed or damaged terminals, poor

terminal-to-wire connection, and damaged harness.

Test Description

Number(s) below refer to the step number(s) on the

Diagnostic Chart:

1. The MIL (“Check Engine” lamp) should be ON

steady with the ignition ON/engine OFF. If not,

isolate the malfunction in the MIL circuit.

2. Checks the serial data circuit and ensures that the

ECM is able to transmit serial data.

6E1-56 DRIVEABILITY AND EMISSIONS

3. This test ensures that the ECM is capable of

controlling the MIL and the MIL driver circuit is not

shorted to ground.

4. If the engine will not start, the Cranks But Will Not

Run chart should be used to diagnose the

condition.

7. A Scan Tool parameter which is not within the

typical range may help to isolate the area which is

causing the problem.

On-Board Diagnostic (OBD) System Check

Step

Action

Value(s)

Yes

1. Ignition ON engine OFF.

2. Observe the malfunction indicator lamp (MIL or

“Check Engine” lamp).

Is the MIL (“Check Engine” lamp) ON?

Go to Step 2

Go to

No MIL

1. Ignition OFF.

2. Install a Scan Tool.

3. Ignition ON.

4. Attempt to display ECM engine data with the Scan Tool.

Does the Scan Tool display ECM data?

Go to Step 3

Go to Step 8

1. Using the Scan Tool output tests function, select MIL

dash lamp control and command the MIL OFF.

2. Observe the MIL.

Did the MIL turn OFF?

Go to Step 4

Go to

MIL(“Check

Engine” Lamp)

On Steady

Attempt to start the engine.

Did the engine start and continue to run?

Go to Step 5

Go to

Cranks But

Will Not Run

Select “Display DTCs” with the Scan Tool.

Are any DTCs stored?

Go to Step 6

Go to Step 7

Are two or more of the following DTCs stored?
• 14
• 21
• 22
• 33
• 34
• 69

Go to

“Multiple ECM

Information

Sensor DTCs

Set”

Go to

applicable

DTC table

Compare ECM data values displayed on the Scan Tool to

the typical engine scan data values.

Are the displayed values normal or close to the typical

values?

Go to Step 8

Refer to

indicated

Component

System Checks

1. Ignition OFF, disconnect the ECM.

2. Ignition ON, engine OFF.

3. Check the serial data circuit for an open, short to

ground, or short to voltage. Also, check the DLC

ignition feed circuit for an open or short to ground and

the DLC ground circuits for an open.

4. If a problem is found, repair as necessary. Was a

problem found?

Go to Step 2

Go to Step 9

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