What does it mean?
OBD II fault code P0743 is a generic code that is defined as “Torque converter clutch (TCC) solenoid –electrical”, and is set when the PCM (Powertrain Control Module) detects an abnormal voltage in the electrical control circuit of the TTC ( T orque C onverter C lutch) solenoid. Note that this code refers to an electrical malfunction of the TTC solenoid or its control circuit, and only very rarely to issues with hydraulic pressure or mechanical failures of an automatic transmission.
Virtually all modern applications use a Torque Converter Clutch that is controlled by the PCM (via a solenoid) to regulate the amount of slippage between the engine and the transmission. Since torque converters depend on the shear strength of transmission fluid to transfer power from the engine to the transmission, some slippage that affects performance and fuel economy negatively is inevitable.
Thus, by using a mechanism that locks the two halves of the torque converter together, a solid link, much like how a clutch on a manual transmission locks the engine to the transmission.
The advantages of this include the facts that by regulating/varying the amount of torque converter lock-up, gear shifts can be made almost seamless, and that on powerful high-end applications, large amounts of slippage can be used to both make gearshifts smooth and comfortable, and to protect transmission components against damage caused by aggressive acceleration.
The image below shows a typical TTC solenoid and how it (and its associated parts) fit into the valve body of a typical automatic transmission.