Scan Tool Diagnostics - Communications

Each module uses information from other vehicle system modules, such as the Powertrain Control Module (PCM), in order to perform some of its functions. Before starting diagnosis, read the circuit information for each module and learn how each module operates and interacts with other vehicle systems. Understanding how the system operates will reduce diagnostic time, avoid misdiagnosis, and prevent unnecessary parts replacement.

If a module losses communications with another module it depends upon for information, the module stores a communications DTC ("U" code prefix). All "U" code DTCs must be diagnosed before diagnosing other DTCs or symptoms.