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Toyota Supra JZA80

Symptom confirmation and diagnostic trouble code check

The diagnostic system in the TOYOTA SUPRA fulfills various functions. The first function is the Diagnostic Trouble Code Check in which a malfunction in the signal circuits to the ECU is stored in code in the ECU memory at the time of occurrence, to be output by the technician during troubleshooting. Another function is the Input Signal Check which checks if the signals from various switches are sent to the ECU correctly. The air conditioning system has an Actuator Check function whereby the ECU automatically operates the actuators of the damper and blowermotor, etc. to check the operation. By using these check functions, the problem areas can be narrowed down quickly and troubleshooting can be performed effectively. Diagnostic functions are incorporated in the following systems in the TOYOTA SUPRA.

System Diagnostic Trouble Code Check Input Signal Check (Sensor Check) Other Diagnosis Function
Engine ○ (with Check Mode) Diagnostic Test Mode
Automatic Transmission ○ (with Check Mode) Diagnostic Test Mode
Anti-Lock Brake
ABS & Traction Control
Supplemental Restraint System
Cruise Control
Air Conditioning Actuator Check

In diagnostic trouble code check, it is very important to determine whether the problem indicated by the diagnostic trouble code is still occurring or occurred in the past but returned to normal at present. In addition, it must be checked in the problem symptom check whether the malfunction indicated by the diagnostic trouble code is directly related to the problem symptom or not. For this reason, the diagnostic trouble codes should be checked before and after the symptom confirmation to determine the current conditions, as shown in the table below. If this is not done, it may, depending on the case, result in unnecessary troubleshooting for normally operating systems, thus making it more difficult to locate the problem, or in repairs not pertinent to the problem. Therefore, always follow the procedure in correct order and perform the diagnostic trouble code check.

Diagnostic trouble code check procedure

Diagnostic trouble code check (Make a note of and then clear) Confirmation of Symptoms Diagnostic Trouble Code Check Problem Condition
Diagnostic Trouble Code Display Problem symptoms exist Same diagnostic trouble code is displayed Problem is still occurring in the diagnostic circuit.
Normal code is displayed The problem is still occurring in a place other than in the diagnostic circuit. (The diagnostic trouble code displayed first is either for a past problem or it is a secondary problem.)
No problem symptoms exist The problem occurred in the diagnostic circuit in the past.
Normal Code Display Problem symptoms exist Normal code is displayed The problem is still occurring in a place other than in the diagnostic circuit.
No problem symptoms exist Normal code is displayed The problem occurred in a place other than in the diagnostic circuit in the past.

Taking into account the above points, a flow chart showing how to proceed with troubleshooting using the diagnostic trouble code check is shown below. This flow chart shows how to utilize the diagnostic trouble code check effectively, then by carefully checking the results, indicates how to proceed either to diagnostic trouble code troubleshooting or to troubleshooting of problem symptoms.

Diagnostic trouble code check


Making a note of and clearing of the diagnostic trouble codes displayed


Symptom confirmation
Problem symptoms exist No problem symptoms exist

Simulation test using the symptom simulation methods

Diagnostic trouble code check

⚫ Diagnostic trouble code displayed
⚫ Problem symptoms exist

⚫ Normal code displayed
⚫ Problem symptoms exist

⚫ Normal code displayed
⚫ No Problem symptoms exist

Troubleshooting of problem indicated by diagnostic rouble code

Troubleshooting of each problem symptom

System Normal
If a diagnostic trouble code was displayed in the initial diagnostic trouble code check, it indicates that the trouble may have occurred in a wire harness or connector in that circuit in the past. Therefore, check the wire harness and connectors.
This guide is based on the book edition Toyota (RM502U, 1997)

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