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

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

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

P0642 is set when the voltage below 4240mV - minimum voltage of sensor power supply from ECM/PCM - is detected for more than 3sec. This code is due to the short to ground in sensor power circuit or the voltage problem inside of ECM/PCM.