An actuator control module is a sub-computer that manages a group of actuators. While the main engine computer handles the big picture, these smaller modules manage local systems. They process sensor data and adjust the actuators in real-time, reducing the computational load on the main engine control module.
A prime example is the HVAC control module. This chip reads temperature sensors inside the cabin and adjusts the dashboard blend door actuators. Another example is the turbocharger control module, which adjusts the electronic wastegate actuator to regulate boost pressure based on throttle load.
If a control module fails, it will lose contact with its actuators. This triggers a communication error code (usually starting with a 'U' on scan tools) and disables the system. Replacing a module often requires programming it with a scan tool to teach it the vehicle's specific options and calibration values.