To access the PCM, connect the Scan Tool to the appropriate DLC. Turn the ignition on to allow the vehicle to identify itself to the Scan Tool. Once communication is established with the PCM, select a function (codes, data, actuator tests, system tests, etc) from the tool main menu. The Scan Tool performs these functions by communicating with the vehicle modules through the data bus circuits.
To read trouble codes without a Scan Tool, cycle the key from "off" to "on" 3 times within 5 seconds. This test can establish whether the PCM is capable of responding to a request. It can also be used if the Scan Tool will not communicate.
The Scan Tool "talks" to the vehicle controllers on separate networks. The tool communicates with the PCM via the SCI Transmit and Receive circuits, and with other modules through the CCD or PCI data bus circuits.