DTC U1800: Can Message From The FICM Is Not Received By The ECM: Description
WARNING: This page is about a different car, the 2002 GMC Sierra, 2002 GMC Cab & Chassis Sierra, 2002 Chevrolet Silverado, and 2002 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 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.