The Check Engine Light may illuminate because of a failed knock sensor. Although the sensor often tests okay, it can fail intermittently, while the vehicle is being driven.
Resources for the Repair and Maintenance of your 2009 Subaru Impreza WRX STI
2009 Subaru Impreza WRX STI
2009 Subaru Impreza WRX STI Problems
Sometimes the Check Engine Light illuminates because the computer detects misfires. Subaru released an improved spark plug wire set to solve the problem. The ignition coil should also be inspected for carbon tracks or short circuits between the contacts.
There is a technical service bulletin on the ABS control module. It is replaceable without having to replace the entire anti-lock brake hydraulic unit. Previously, the ABS control module was only available as a complete assembly that included the hydraulic unit.