The electronic throttle control system controls the throttle valve opening. The system includes the throttle actuator, the throttle valve, throttle position (TP) sensors A and B, the PGM-FI subrelay circuit, the accelerator pedal position (APP) sensor, and the powertrain control module (PCM). When the accelerator pedal is pressed, the APP sensor detects the accelerator pedal opening value. The accelerator pedal opening value is converted to a signal in the APP sensor and transmitted to the PCM to compute the target position. The PCM determines the throttle valve target position according to the signal received and operates the throttle actuator to move the throttle valve to the target position. The actual throttle valve position is determined by TP sensors A and B (installed in the throttle body). The PCM transmits a signal to the throttle actuator and moves the throttle valve to the fully closed position to register the throttle valve fully closed position after the vehicle is turned to the OFF (LOCK) mode. When the registration of the throttle valve fully closed position is not completed within a predetermined time after the vehicle is turned to the OFF (LOCK) mode, also the registered value is out of predetermined range, the PCM detects a malfunction in the electronic throttle control system and stores a DTC.