Use a scan tool (Tech 2) to read the status of inputs and outputs for the class 2 serial data line (the BCM is not part of the class 2 line). Use this information to check that the module is sending and receiving the proper data. Each module uses data received from other vehicle systems in order to carry out some functions. Therefore, if a module receives the incorrect data from another vehicle system, the module may function improperly. Use the scan tool in order to review the data coming from its inputs and vehicle systems that interact with that module.

Use a scan tool to cycle outputs. Refer to the scan tool user's manual for menu navigation instructions.

Use this feature to determine if the module is able to activate an output, regardless of inputs. Use a scan tool to select an output and command the module to activate the output. The scan tool then displays the status of the selected output. The status information represents only what the module has commanded, not what action was actually taken. Check if the module is performing each of the selected output functions.

Each module uses information from other vehicle systems, 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.

Use a scan tool (Tech 2) to diagnose the module when specified in the diagnostic tables. A scan tool displays Diagnostic Trouble Codes (DTCs), the status of inputs and outputs, and commands outputs on and off.

Become very familiar with the capabilities and diagnostic features. These self-diagnostic features are referenced in the diagnostic tables and are very useful in diagnosing non-DTC setting malfunctions as well.

Replace the module only when a diagnostic table specifically direct you to do so. Inputs and outputs are protected against shorts to battery or ground. Therefore, do NOT replace the module after repairing a short in the wiring. After repairing the wiring, perform the diagnostic system check to verify the repair. Most malfunctions related to a vehicle system, such as the IP, are traceable to faulty wiring, connectors and terminals, and occasionally some other component.

Each module is very reliable and it is not likely the cause of the malfunction. Replacing the module before a complete diagnosis will usually result in a recurrence of the original malfunction.

Each class 2 module stores DTCs as current or history DTCs. If the module detects a malfunction, it stores a DTC that corresponds to that particular malfunction. If the module stores a current DTC and the malfunction is no longer present, some DTCs will remain current until the next ignition cycle. Other DTCs change from current to history without requiring a new ignition cycle when the fault is no longer present. History DTCs are intermittent malfunctions and must be diagnosed differently than current DTCs. Refer to Intermittents and Poor Connections Diagnosis for more information.