An electronic control system in which a sensor transmits a data to a control unit is provided. In one embodiment, the sensor includes a first detector and a second detector each converting a physical quantity of a detection target into a digital data. An output portion of the sensor outputs in turn a normal data and a monitor data associated with each other. The normal data is the digital data outputted from the first detector, and the monitor data is a reversed data of the digital data outputted from the second detector. The control unit determines that at least one of the first detector, the second detector and the output portion has failure when the reversed data does not match the normal data.