Intermittent Concerns — How to Find Them

Intermittent concerns that cannot be duplicated are among the hardest diagnostic challenges in the shop. The concern is real — the customer is not making it up. It existed at some point. Your job is to create the conditions that cause it to occur.

Hot or cold engine? Which direction was the vehicle traveling? Highway or stop and go? Going uphill or downhill? After how many minutes of driving? Over a bump? After a specific accessory was used? The more precisely the customer can describe the conditions, the better your chance of duplicating it. Ask very specific questions about everything that was happening when the concern occurred.

When the PCM sets a fault code, it records a freeze frame — a snapshot of all sensor data at the exact moment the fault occurred. Engine load, RPM, vehicle speed, coolant temperature, fuel trims. This snapshot tells you the exact operating conditions when the fault happened. Use it to recreate those exact conditions during your test drive.

For intermittent electrical concerns, with the system activated, wiggle every harness and connector in the relevant circuit while monitoring for the fault. Move the harness along its entire length. Flex the connectors. The fault that appears when you wiggle a specific section identifies exactly where the intermittent connection is.