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

History DTCs are those indicating that the module has previously detected a malfunction. The reason could be either that the malfunction is intermittent or that the system for which the DTC is set for is not currently being operated. For example, the rear defogger circuit does not operate until the VCM/PCM detects engine RPM. For this reason, if a module displays history DTCs and the malfunction cannot be found easily, try clearing the DTCs and manually operating 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 but do not necessarily indicate a faulty module. The module are reliable components, and are not a likely cause of the malfunction. Most system complaints are linked to faulty wiring and connectors, and occasionally to a component. Perform a thorough inspection of all 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: