Often known as a gearbox control unit (GCU) or a transmission control module (TCM), a transmission control unit a type of automotive ECU that controls electronic automatic transmissions. It makes use of sensors from the vehicle together with the ECU (engine control unit) data, to determine when and how to change gears in the vehicle to get the best possible performance, shift quality, and fuel economy.

Signs of a bad transmission control unit:

• Check Engine Light
• Unpredictable Shifting
• Problems Shifting into Higher Gears
• Problems Downshifting
• Stuck in the Same Gear
• Delayed Shifting
• Poor Fuel Economy