Circuit Description
WARNING: This page is about a different car, the 2003 GMC Sierra, 2003 GMC Cab & Chassis Sierra, 2003 Chevrolet Silverado, and 2003 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The engine control module (ECM), fuel injection module (FICM), and the transmission control module (TCM) communicate control and diagnostic information via a SAE J1939 CAN communication bus. The ECM monitors CAN operational status by expecting a constant flow of messages from the FICM and the TCM. If the ECM fails to receive an expected message from one of the other modules, DTC U1800, U2104, or U2106 will set, depending on what communication is lost