Since the PCM is programmed to recognize the presence and value of electrical inputs, it will also note the lack of a signal or a radical change in values. It will, for example, react to the loss of signal from the vehicle speed sensor or note that engine coolant temperature has risen beyond acceptable (programmed) limits. Once a fault is recognized, a numeric code is assigned and held in memory. The dashboard warning lamp: CHECK ENGINE or SERVICE ENGINE SOON (SES), will illuminate to advise the operator that the system has detected a fault. This lamp is also known as the Malfunction Indicator Lamp (MIL).
More than one code may be stored. Keep in mind not every engine uses every code. Additionally, the same code may carry different meanings relative to each engine or engine family.
In the event of an computer control module failure, the system will default to a pre-programmed set of values. These are compromise values, which allow the engine to operate, although possibly at reduced efficiency. This is variously known as the default, limp-in or back-up mode. Driveability is almost always affected when the PCM enters this mode.
On 1990-95 models, the stored codes may be read with only the use of a small jumper wire, however the use of a hand-held scan tool such as GM's TECH-1® or equivalent is recommended. On all 1996-00 models, an OBD-II compliant scan tool must be used. There are many manufacturers of these tools; a purchaser must be certain that the tool is proper for the intended use. If you own a scan type tool, it probably came with comprehensive instructions on proper use. Be sure to follow the instructions that came with your unit if they differ from what is given here; this is a general guide with useful information included.
The scan tool allows any stored codes to be read from the PCM memory. The tool also allows the operator to view the data being sent to the computer control module while the engine is running. This ability has obvious diagnostic advantages; the use of the scan tool is frequently required for component testing. The scan tool makes collecting information easier; an operator familiar with the system must correctly interpret the data.
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 normal values or a known good vehicle reveals the problem quickly.
The most commonly required electrical diagnostic tool is the digital multimeter, allowing voltage, ohmage (resistance) and amperage to be read by one instrument. The multimeter must be a high-impedance unit, with 10 megohms of impedance in the voltmeter. This type of meter will not place an additional load on the circuit it is testing; this is extremely important in low voltage circuits. The multimeter must be of high quality in all respects. It should be handled carefully and protected from impact or damage. Replace batteries frequently in the unit.
Other necessary tools include an unpowered test light, a quality tachometer with an inductive (clip-on) pick up, and the proper tools for releasing GM's Metri-Pack, weather Pack and Micro-Pack terminals as necessary. The Micro-Pack connectors are used at the PCM electrical connector. A vacuum pump/gauge may also be required for checking sensors, solenoids and valves.