The ECU is equipped with a built-in self diagnostic system, which monitors the speed sensors, solenoid valves and their electrical circuitry. If the ECU senses a malfunction:
1. It blinks the OD OFF light to warn the driver.
2. It stores the malfunction code in its memory.
3. (When properly accessed) it will output a diagnostic code indicating the faulty component or circuit.
Once a malfunction is stored in the memory system, it will be retained until canceled (erased). The vehicle battery constantly supplies 12 volts to the ECU B terminal to maintain memory even if the ignition switch is turned off. If the malfunction is repaired or returns to normal operation, the warning light will go off but the malfunction code will remain in memory. In order to erase a diagnostic code from the memory, a specified fuse must be removed for approximately 30 seconds with the ignition switch is off. The fuse is identified in the repair manual or on the ECT Diagnostic Information technician reference card.
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