Why could be wrong with my car?

It's not the torque converter itself that is bad, it is the solenoid that applies it. When the solenoid goes bad, it doesn't release. Open the hood, look down at the front side of the transmission, and locate the four wire plug connected to the transmission. Unplug it. Now your torque converter clutch will not lock up. The solenoid is serviced thru the side cover on the trans, replacement should cost $250 to $300.