Theory Of Operation

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Sensor information and other calculated data that is processed by an Electronic Control Module (ECU) can be transmitted over the Controller Area Network (CAN). Some ECUs require information from other ECUs to check the validity of the information relative to that system. In some cases that information is used in the calculations relative to that ECU.

If the transmitting ECU knows that the data is not valid, it will transmit a CAN message indicating that the information is not valid or not available. Often, this message is known as a Signal Not Available (SNA) message.

When a SNA message is transmitted, a Diagnostic Trouble Code (DTC) will be set in the receiving ECU.

If a SNA message is received, this indicates that the transmitting ECU cannot transmit good data relative to the information that it was supposed to send. Typically, the transmitting ECU will also have a DTC set related to that information. Therefore, in most cases, this DTC is considered informational because the error is related to another ECU.

The Electronic Shift Module (ESM) performs various internal tests to verify proper controller operation. This DTC indicates there is a communication issue between the ESM and Transmission Control Module (TCM).