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Home >> Toyota Truck >> 1996 >> Previa Van 4WD L4-148.8 2438cc 2.4L DOHC SC (2TZ-FZE) >> Repair and Diagnosis >> Powertrain Management >> Computers and Control Systems >> Testing and Inspection >> Initial Inspection and Diagnostic Overview >> Diagnostic Strategies >> How to Proceed With Troubleshooting

How to Proceed With Troubleshooting

The Engine Control System broadly consists of the sensors, engine control module and actuators. The engine control module receives signals from various sensors, judges the operating conditions and determines the optimum injection duration, timing, ignition timing and idle speed.
In general, the Engine Control System is considered to be a very intricate system to troubleshoot. But, the fact is that if you inspect each circuit in order following the procedures in this manual, troubleshooting of this system is not complex.
When troubleshooting OBDII vehicles, use an OBDII scan tool complying with SAE J1978 or TOYOTA hand-held tester to confirm the diagnostic trouble codes, freeze frame data and engine data. This will enable you to determine the system causing the problem.





The following section explains the best method of troubleshooting and how to carry Out the necessary repairs.

1. CUSTOMER PROBLEM ANALYSIS
Using the customer problem analysis check sheet for reference, ask the customer in detail about the problem.
2. CONNECT OBDII SCAN TOOL OR TOYOTA HAND-HELD TESTER TO DLC3
Connect the OBDII scan tool complying with SAE J1978 or TOYOTA hand-held tester to the vehicle's data link connector 3.

NOTE: For OBDII scan tool or TOYOTA hand-held tester operating instructions, see the instruction booklet accompanying the scan tool or tester.

If your display shows "UNABLE TO CONNECT TO VEHICLE" when you have connected the scan tool/TOYOTA hand-held tester cable to DLC3, turn the ignition switch ON and operate the scan tool/TOYOTA hand-held tester, inspect DLC3.
3. CHECK DIAGNOSTIC TROUBLE CODE AND FREEZE FRAME DATA (PRECHECK)
First check the diagnostic trouble codes. If a code is output, make a note of it. Also check and note the freeze frame data.

HINT: Output of the malfunction code indicates a circuit malfunction. However, it does not indicate whether the malfunction is still occurring or occurred in the past and returned to normal. To determine this, first confirm the problem symptoms in step 7 and then recheck the diagnostic trouble code in step 9.
If troubleshooting started based only on the malfunction code in the diagnostic trouble code check in step 3, it could result in a misdiagnosis and troubleshooting of circuits which are normal, making it more difficult to locate the cause of the problem.
4. CLEAR DIAGNOSTIC TROUBLE CODE AND FREEZE FRAME DATA
Use the OBDII scan tool or TOYOTA hand-held tester to erase the diagnostic trouble codes and freeze frame data.

NOTE: For OBDII scan tool or TOYOTA hand-held tester operating instructions, see the instruction booklet accompanying the scan tool.

5. VISUAL INSPECTION
6. SETTING CHECK MODE DIAGNOSIS
7. PROBLEM SYMPTOM CONFIRMATION
If the engine does not start, first carry out steps 10 and 12 while referring to the diagnostic trouble codes confirmed in step 3.
8. SYMPTOM SIMULATION
To find the trouble more quickly, set the diagnosis check to check mode and confirm the problem symptoms with the higher sensing ability of the engine control module. If the trouble does not reappear, use the symptom simulation method to make sure the trouble can be reproduced.
9. DIAGNOSTIC TROUBLE CODE CHECK IN CHECK MODE
Check the diagnostic trouble code in check mode. If a malfunction code is output, proceed to step 11. If a malfunction code is not output, proceed to step 10.
10. BASIC INSPECTION
Carry out basic inspection such as the spark check and fuel pressure check, etc.
11. DIAGNOSTIC TROUBLE CODE CHART
If a malfunction code is displayed, inspect the circuit indicated by the chart for each code.
12. DIAGNOSIS BY SYMPTOM
If a diagnostic trouble code is not displayed in the diagnosis in check mode, troubleshoot according to the inspection order in DIAGNOSIS BY SYMPTOM.
13. CIRCUIT INSPECTION
Determine if the malfunction is in the sensor, actuator, wire harness, connector or the engine control module.
14. PARTS INSPECTION
When the Matrix Chart of Problem Symptoms instructs you to check the parts, refer to the parts inspection section in this manual.
15. CHECK FOR INTERMITTENT PROBLEMS
By checking for intermittent problems, you can isolate the place where momentary interruptions or momentary shorts are occurring due to poor contacts.
16. ADJUSTMENT, REPAIR
After you locate the cause of the problem, follow the inspection and replacement procedures in this manual and adjust or repair as necessary.
17. CONFIRMATION TEST
After completing adjustment or repairs, confirm not only that the malfunction is eliminated, but also test drive the vehicle, to make sure the entire Engine Control System is operating normally.

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