For 1990-2009 cars

The engine control module (ECM), transaxle control module (TCM), and body control module (BCM) communicate normal mode messages over the controller area network (CAN) link circuits 2500 and 2501. Each controller on the CAN link uses a variable pulse width modulated square wave consisting of logic 0's and logic 1's. A logic 0 is denoted as circuit 2501 at 1.5 volts with circuit 2500 at 3.5 volts resulting in a difference of 2 volts across the circuits. A logic 1 is denoted as both circuit 2500 and 2501 at 2.5 volts resulting in a difference of 0 volts across the circuits.

State of health messages are sent from each CAN controller on the link when the ignition is turned ON as well as during normal vehicle operation. These messages allow the BCM, ECM, and TCM to know that each controller on the link is functioning correctly. The ECM monitors the CAN link for low and high voltage faults. All controllers on the CAN link monitor each others messages and can set a DTC if information is not received. DTC U2104 will set if a specific controller has to reset its CAN bus counter a certain number of times due to a controller not communicating.

### DTC Parameters

DTC U2104 will set if any of the modules tied to the CAN link, BCM, ECM, or TCM, does not receive messages from another control module on the CAN link when:

• | The condition exists for longer than 2 seconds for the TCM, 3 seconds for the BCM, and 25 seconds for the ECM

• | The ignition is ON for the BCM and TCM and the engine is running for the ECM.

DTC U2104 runs continuously with the ignition ON for the BCM and TCM, or the engine running for the ECM. The ECM will PASS the diagnostic in 2 seconds with the engine running if the fault is no longer present.

DTC U2104 is a type A DTC for the ECM and TCM.

DTC U2104 is a type C DTC for the BCM.

### Diagnostic Aids

DTC U2104 may set along with U2105, U2106, or U2107 if a specific controller is not communicating over the CAN link.

The TCM will command maximum line pressure to the transaxle if this DTC is active and the ECM is not communicating to the TCM.

Perform the Vehicle On-board Diagnostic System Check in this section.