On-Board Diagnostics For The Communication Links

CLASS II
Each Class II module is programmed with a list of serial data messages that are important and should be received by the module. One of those important messages is the "State Of Health" (SOH) message. The SOH message is sent out periodically, even if no other information is being transmitted. The SOH message is used to let all other modules on the serial data circuit know the sending module is working correctly.

Each controller relies on information from another controller and incorporates a diagnostic associated with that controller. The diagnostic uses the SOH message for performing the diagnostic. Each time the ignition is turned On, Class II modules first learn the other modules connected to the Class II serial data circuit. If a controller did not initially receive needed information when the ignition was first turned On, the controller will not know which controller the needed information comes from. In this case, DTC U1000 will be set, indicating a message was not received. Once a controller has learned which controller has sent the information, a DTC indicating which controller stopped communications will be indicated if that message is lost. Each controller learns which other controller is sending the message it has to receive every ignition cycle.

Refer to specific U code definitions for more details of code setting conditions.