Possible causes

– Faulty Electronic Brake Control Module (EBCM)
– EBCM harness is open or shorted
– EBCM circuit poor electrical connection

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

U0121 GMC Description

Modules connected to the high speed General Motor Local Area Network (GMLAN) serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. The modules have prerecorded information about what messages are needed to be exchanged on the serial data circuits, for each virtual network. The messages are supervised and also, some periodic messages are used by the receiver module as an availability indication of the transmitter module. The supervision time-out period is 250 ms. Each message contains the identification number of the transmitter module.

GMC Technical Service Bulletins for U0121 code