A converter control system includes first and second step-up converters and first and second electronic control units. The first and second electronic control units are respectively configured to generate first and second PWM driving signals for the first and second step-up converters. The second electronic control unit is configured to determine whether a first carrier and a second carrier are synchronous with each other based on the first PWM driving signal, a first duty ratio, and the second carrier. The second electronic control unit is configured to, when the second electronic control unit determines that the first carrier and the second carrier are not synchronous with each other, shut down the second step-up converter and then restart generation of the second PWM driving signal with the second carrier synchronous with the first carrier with the use of the first PWM driving signal and the first duty ratio.