U1800

CIRCUIT DESCRIPTION
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.

CONDITIONS FOR RUNNING THE DTC
The ignition is ON.

CONDITIONS FOR SETTING THE DTC
The CAN message from the FICM is not received by the ECM for 45 milliseconds.

ACTION TAKEN WHEN THE DTC SETS
^ The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
^ The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
^ A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
^ A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
^ Clear the MIL and the DTC with a scan tool.

DIAGNOSTIC AIDS
The modules that communicate on the CAN system are wired parallel to each other until the respective circuits are spliced together for the ECM. An open in the CAN circuits of one module will not affect the other module. A short to ground or short to voltage affects all modules no matter where the failure occurs.

TEST DESCRIPTION

DTC U1800 - Steps 1-5:

DTC U1800 - Steps 6-14:

DTC U1800 - Steps 15-24:

The numbers below refer to the step numbers on the diagnostic table.
2. If the engine does not start and the system sets DTC U1800, the fault lies in the FICM or the FICM power and ground circuits.
3. If the fault is a short to ground, a short together, or an open between the ECM and the CAN circuit splices, DTC U2104 will also set. These failures effect the entire system.
4. If only DTC U1800 and U2106 set, The problem is a short to voltage in the CAN high circuit. This failure does not set DTC U2104.