Use a scan tool in order to diagnose the control 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 familiar with the scan tool operation, capabilities and diagnostic features. The diagnostic features are used in the diagnostic trouble code (DTC) diagnostic tables and are very useful in diagnosing non-DTC malfunctions.

Each module uses information from other vehicle system modules, such as the powertrain control module (PCM), in order to perform some of the 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 that the module depends upon for information, the module stores a communications DTC ("U" code prefix). Diagnose all "U" code DTCs before diagnosing other DTCs or symptoms.

Use a scan tool in order to read the status of inputs and outputs. Use this information in order to verify that the module is sending and receiving the proper data. Each module uses data received from other vehicle modules in order to carry out some system functions. Therefore, if a module receives incorrect data from another vehicle module, the original module and system tested may function improperly. Use the scan tool in order to review the data coming from the module inputs and from other vehicle systems and modules that interact with that module.

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

Use this feature in order to determine if the module is able to activate an output. Use a scan tool in order to select an output and command the module to activate the output. The scan tool then displays the status of the selected output. This status information represents only what the module has commanded, not what action was actually taken. Inspect the rest of the output circuitry to see if the module is performing the selected output functions.

Each module stores DTCs as current or history DTCs. If the module detects a malfunction, the module stores a DTC that corresponds to that particular malfunction. The DTC diagnostic tables will find the cause of the malfunction.

Some modules set a current DTC, but the malfunction is no longer present. The current DTC status indicates that the module requires an ignition switch cycle in order to either clear the DTC or in order to store the DTC as a history DTC.

Some DTCs will remain current until the next ignition switch cycle. Other DTCs change from current to history without requiring a new ignition switch cycle when the fault is no longer present. History DTCs are intermittent malfunctions. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems for more information on diagnosing intermittent concerns.

Replace the module only when a diagnostic table specifically directs you to do so. Inputs and outputs are protected against a short to battery or a short to ground. Do NOT replace the module after repairing a short in the wiring. Most malfunctions related to a vehicle system are traceable to faulty wiring, connectors and terminals, and occasionally to some other component in the system.

Each module is very reliable and 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.