The gear box ECU (sometimes known as a Transmission Control Unit) is an essential part of the gear box in an automatic vehicle, and monitors and controls the gear changes and transmission. This electrical part communicates with the engine ECU using a series of sensors that are positioned at strategic points in the vehicle. Using the information given to it by the sensors and the engine ECU it calculates when to change the vehicle’s gears to improve vehicle performance and minimise fuel consumption. These sensors tell the gear box ECU what speed the vehicle is driving at, whether it is driving uphill or downhill, the temperature of the oil and the amount of torque generated by the torque converter.

Some vehicles have a combined gear box ECU and engine ECU, known as a powertrain control module (PCM).

Damage to the gear box ECU is likely down to the solenoids – magnetic wire coils that transport electric current. If your vehicle experiences problems with the gear box ECU you may first notice a warning code displayed in your vehicle. If ignored, the gear box can fail resulting in an inability to change gears.