Scan Tools

All late-model vehicles utilize computer modules to monitor and control the functions of on-board systems. These modules are known by many names such as Engine Control Unit (ECU), Engine Control Module (ECM), Powertrain Control Module (PCM) and Vehicle Control Module (VCM) just to name a few. When problems occur in control circuits, these modules record a diagnostic trouble code which can be used to help solve the problem. Over the years, there have been many different types of systems, each with their own unique way of retrieving these codes. On a good number of the older systems, the stored codes were flashed on various trouble lights (found in the dashboard) once a small jumper wire was placed across the proper diagnostic terminals. However the use of a hand-held scan tool was still preferred for these systems.

For some models produced during the 1995 model year and on almost every single 1996 and later model, a new form of trouble codes was developed which required the use of a scan tool. On Board Diagnostic-II (OBD-II) compliant vehicles use a 5 digit, alpha-numeric code which would be difficult or impossible to read using a flashing light, therefore trouble code reading on an OBD-II compliant requires a scan tool.

There are many manufacturers of these tools, but 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

The scan tool allows any stored codes to be read from the computer module 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; the data must be correctly interpreted by an operator familiar with the system.

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 computer module 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 module may be viewed; comparison to normal values or a known good vehicle reveals the problem quickly.