Intermittent failures in electronic systems may be very difficult to detect and to accurately diagnose. The modules test for different malfunctions under different vehicle conditions. Thoroughly test drive the vehicle in order to repeat a fault condition. If the system malfunction is not repeated during the test drive, obtain a good description of the complaint. This description may be useful in locating an intermittent malfunction. Faulty wires or connections cause most intermittent conditions. When an intermittent condition is suspected, inspect the suspected circuits for the following conditions:

History diagnostic trouble codes (DTCs) indicate that the modules have previously detected a malfunction. The reason may be one of the following conditions:

For example, the rear defogger circuit does not operate until the modules detect engine revolutions per minute (RPM). If a module displays history DTCs and the malfunction cannot be found easily, perform the following steps:

1. Clear the DTCs.
2. Manually operate the system that the DTC applies to using a scan tool.

Read the diagnostic aids and conditions for setting the DTC in each DTC table for more specific information.

History DTCs help in diagnosing intermittent fault conditions. History DTCs do not necessarily indicate a faulty module. The modules are reliable components and are not likely the cause of the malfunction. Most system complaints are linked to faulty wiring and connectors. Occasionally a system complaint is linked to a component. Perform a thorough inspection of all of the related wiring and connectors pertaining to the history DTC that is stored. The following conditions may result in intermittent module operation with no DTC stored: