From the code, I assume your car is an automatic. If the car drives and shifts all right, then this code does not indicate that the automatic's torque converter isn't operating properly. It most likely means that a sensor in the transmission is shorted or malfunctioned. If this sensor is giving faulty information to the engine, it might effect the way the car idles, especially when you are stopped with the engine running and your foot on the brake. If you aren't hearing any odd engine behavior, then you probably aren't doing any damage. Depending on the state where you live, you might have trouble updating your registration. In California, if the engine check light is on, the car won't pass smog.
I have a 1999 BMW 328I with 104,000 miles and the Engine Management Light came on. I recently purchased a OBD II Code reader and the Code P0740 came up. This is what it reads " Torque Converter Clutch Circuit/Open". Does anyone have any clue what the fix is or the extent of damage that can be done to the car if it doesn`t get fixed soon?