Limitations

In the generally used special or normal test modes the SCAN tool allows a quick check of sensors and switches which provide inputs to the ECM, but on models with a 160 baud ECM, the data only updates once every 1.25 seconds making the tool less effective than a voltmeter for locating intermittent conditions which may last for less than 1.25 seconds. However, the SCAN tool does allow the technician to manipulate wiring harnesses or underhood components while observing the SCAN readout. This aids in locating intermittent malfunctions while the engine is not running.
It is also important to note that while the SCAN tool may indicate that a component has been activated by the ECM, this indication only represents the ECM command to the specific circuit. The only way to verify that the command has been transmitted and carried out is to observe operation of the specific component activated by the ECM, or by noting changes in the system that the device controls. However, the SCAN tool allows the technician to compare operating parameters of a poorly operating engine with the parameters of an engine which is operating properly. Therefore, system malfunctions that may not ordinarily set a trouble code in the ECM memory, such as a shift in sensor values or excessive resistance in an operating element, can be uncovered by comparing operation of like systems.
The SCAN tool is capable of testing a variety of electronic engine control systems, and therefore not all tester positions will be applicable for each engine/ECM combination being tested. If a position is selected that is not applicable to the particular combination, this condition will be indicated on the tester. Refer to the manufacturer's instructions for information regarding the SCAN tester positions and how they can aid diagnosis.