Most modern vehicles use a Torque Converter Clutch (TCC) which is controlled by the Power Train Control Module (PCM) by means of a solenoid, this is in order to regulate the amount of slip that may have between the engine and the transmission.
The design of the TCC is made for better transmission efficiency during high speeds. Also, control the heat.
Diagnostic code P0743 OBD II is set when the PCM detects a voltage outside the expected range in the electrical control circuit of the TCC. In general, this code is generated by an electrical fault of the TCC solenoid or its control circuit.