Modules connected to the high speed GMLAN serial data circuits monitor for serial data communications on the high speed GMLAN network during normal vehicle operation. Operating information and commands are exchanged among the modules. When a module detects a bus-off condition a DTC U0001, U2100 or U2104 will be set. Generally these DTCs can be retrieved as history only, with the exception of body control module (BCM) that can set DTC U2100 as current, because it communicates with the scan tool on class 2.

The high speed GMLAN serial data bus uses two 120 ohms terminating resistors that are in parallel with the high speed GMLAN (+) and (-) circuits. One of the resistors is connected at the body control module (BCM) and the other is at the engine control module (ECM). If testing for a short between high speed GMLAN (+) and (-) a reading of 60 ohms is normal. If the high speed GMLAN serial data is open testing the resistance between high speed GMLAN (+) and (-) will read about 120 ohms.

The engine will not start when there is a total malfunction of the high speed GMLAN serial data circuits while the engine is not running. The following conditions may cause a total loss of high speed GMLAN data communication: