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 the other modules on the serial data circuit know the sending module is working correctly.

Each controller that 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 all the modules on the Class II serial data circuit first learn the other modules connected to the Class II serial data circuit. If a controller relies on information, but it is not received, the controller will not know what other device was supposed to send the information. In this case a 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 will learn 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.

CAN LINK DIAGNOSTICS
Diagnostics on the CAN link are similar to Class II. The main difference is that the source of information a controller relies on is always known, and therefore, a specific code indicating which controller may have stopped communicating is always known.

KWP2000 DIAGNOSTICS
Since this link is only used for diagnostics/reprogramming, and not used for normal vehicle communications, there are no on-board diagnostics associated with this link.