ECM is activated by power supply. Signals from several sensors, such as CKPS and APS, is inputted to ECM. Comparing inputted siganals with control LOGIC saved at micro controller and EEPROM, ECM controlls engine as actuating injectors, solenoids and relays.

To guarantee accurate control, ECM performs SELF TEST, DIAGNOSIS of several sensors and actuators. And if serious trouble which affects vehicle performance occurrs, ECM sets DTCs.

At certain cases, ECM shuts down whole systems in order to prevent dangerous situation due to incorrect control.

P0643 is set when the voltage above 5.75V - maximum voltage of sensor power supply from ECM - is detected for more than 3 sec. This code is due to the short to battery in sensor power circuit or the voltage problem inside of ECM.