15.1 ELECTRONIC ENGINE CONTROL SYSTEM
When the electronic engine control system detects a fault, it broadcasts a message on the datalink. The dash display will show the code "ECU 128," indicating there is an engine fault.
To read the complete fault code, refer to the MBE Electronic Controls Troubleshooting Guide (6SE422) or bring the vehicle to an authorized repair facility. Complete fault codes can be read using minidiag2.
Basically they are saying that the problem may be with the actual computer it's self. Most other codes would refer to items attached to the engine but not internal to the computer.