On the vehicles covered by this manual, the DTCs can only be read using a scan tool, such as GM's Tech 1® or Tech 2®, or equivalent. These tools are so-called OBD-II Compliant. This means they are compatible with the federal-mandated On-Board Diagnostic, Second Generation systems. Almost from the beginning of computer controlled vehicles, some sort of on-board diagnostic ability has been built into the vehicle's computer to assist the technician. At one time, the vehicle's instrument panel Malfunction Indicator Lamp could be used to read some codes. Even a vehicle owner could read the flashing MIL lamp and interpret the trouble code. Scan tools became available to the profession technician. A scan tool allows any stored codes to be read from the PCM memory. The tool also allows the technician to view the data being sent to the PCM while the engine is running. While the scan tool makes collecting information easier, the data must be correctly interpreted by a technician familiar with the system.
As emission regulations get more stringent and vehicles get more complex, the vehicle's on-board computers have evolved into powerful diagnostic tools. The vehicles covered by this manual require the use of OBD-II compliant Scan Tools for efficient troubleshooting, reading DTCs and reading the PCM's data stream. These vehicles use what is called a Class II Serial Data circuit to a Diagnostic Link Connector which allows bi-directional communication between the PCM and the scan tool.
An example of the usefulness of the scan tool may be seen in the case of a temperature sensor which has changed its electrical characteristics. The PCM is reacting to an apparently warmer engine (causing a driveability problem), but the sensor's voltage has not changed enough to set a fault code. Connecting the scan tool, the voltage signal being sent to the PCM may be viewed. Comparison to either factory published information of normal values or a known good vehicle may reveal the problem quickly.