Scan Tool Usage With Intermittents

SCAN TOOL USAGE WITH INTERMITTENTS
In some Scan tool applications, the data update rate makes the tool less effective than a voltmeter. such as when trying to detect an intermittent problem which lasts for a very short time. However. the Scan tool does allow one to manipulate wiring harnesses or components under the hood with the engine not running, while observing the readout of the Scan tool.

Scan tool display can be observed while driving the vehicle under the condition when the malfunction indicator lamp (MIL) or SERVICE telltale turns On or when engine driveability is momentarily poor. An assistant should be used when driving a vehicle and using a Scan tool. If the problem seems to be related to certain parameters that can be checked on the Scan tool, they should be checked while driving the vehicle. If there does not seem to be any correlation between the problem and any specific circuit. the Scan tool can be checked at different conditions watching for a period of time to see if there is any change in the readings that would indicate intermittent operation.

The Scan tool is also an easy way to compare the operating parameters of a poorly operating engine with those of a known good one. For example, a sensor may shift in value, but not set a diagnostic trouble code. Comparing sensor readings with those of a known good vehicle may uncover the problem.

The ECM has the ability to take a snapshot when a diagnostic trouble code is stored. This information can be found with a Scan tool listed under Failure Record information. This information can be very useful when attempting to troubleshoot intermittent problems.

VEHICLE ON-BOARD DIAGNOSTIC SYSTEM CHECK
After the visual/physical underhood inspection, the vehicle on-board diagnostic (OBD) system check is the starting point for all diagnostic procedures or finding the cause of an emissions test failure.

The correct procedure to diagnose a problem is to follow three basic steps:
1. Are the on-vehicle diagnostics working? This is determined by performing the vehicle on-board diagnostic (OBD) system check. Since this is the starting point for the diagnostic procedures or finding the cause of an emissions test failure, always begin here. The "Vehicle On-Board Diagnostic System Check" procedure can be found in the Data Link Communications section of the Body Electrical Diagnostics service manual.
2. Is there a diagnostic trouble code stored? If a diagnostic trouble code is stored, go directly to the numbered diagnostic trouble code chart. This will determine if the fault is still present. If no diagnostic trouble code is stored, proceed to Step 3.
3. Scan serial data transmitted by the ECM. This involves reading the information available on the serial data stream with a Scan tool. The meaning of the various displays can be found in the Scan tool data definition section. Expected readings under a particular operating condition can be found in the Scan tool data definition section.